1// Copyright 2009 The Go Authors. All rights reserved. 2// Use of this source code is governed by a BSD-style 3// license that can be found in the LICENSE file. 4 5// Garbage collector (GC). 6// 7// The GC runs concurrently with mutator threads, is type accurate (aka precise), allows multiple 8// GC thread to run in parallel. It is a concurrent mark and sweep that uses a write barrier. It is 9// non-generational and non-compacting. Allocation is done using size segregated per P allocation 10// areas to minimize fragmentation while eliminating locks in the common case. 11// 12// The algorithm decomposes into several steps. 13// This is a high level description of the algorithm being used. For an overview of GC a good 14// place to start is Richard Jones' gchandbook.org. 15// 16// The algorithm's intellectual heritage includes Dijkstra's on-the-fly algorithm, see 17// Edsger W. Dijkstra, Leslie Lamport, A. J. Martin, C. S. Scholten, and E. F. M. Steffens. 1978. 18// On-the-fly garbage collection: an exercise in cooperation. Commun. ACM 21, 11 (November 1978), 19// 966-975. 20// For journal quality proofs that these steps are complete, correct, and terminate see 21// Hudson, R., and Moss, J.E.B. Copying Garbage Collection without stopping the world. 22// Concurrency and Computation: Practice and Experience 15(3-5), 2003. 23// 24// 1. GC performs sweep termination. 25// 26// a. Stop the world. This causes all Ps to reach a GC safe-point. 27// 28// b. Sweep any unswept spans. There will only be unswept spans if 29// this GC cycle was forced before the expected time. 30// 31// 2. GC performs the mark phase. 32// 33// a. Prepare for the mark phase by setting gcphase to _GCmark 34// (from _GCoff), enabling the write barrier, enabling mutator 35// assists, and enqueueing root mark jobs. No objects may be 36// scanned until all Ps have enabled the write barrier, which is 37// accomplished using STW. 38// 39// b. Start the world. From this point, GC work is done by mark 40// workers started by the scheduler and by assists performed as 41// part of allocation. The write barrier shades both the 42// overwritten pointer and the new pointer value for any pointer 43// writes (see mbarrier.go for details). Newly allocated objects 44// are immediately marked black. 45// 46// c. GC performs root marking jobs. This includes scanning all 47// stacks, shading all globals, and shading any heap pointers in 48// off-heap runtime data structures. Scanning a stack stops a 49// goroutine, shades any pointers found on its stack, and then 50// resumes the goroutine. 51// 52// d. GC drains the work queue of grey objects, scanning each grey 53// object to black and shading all pointers found in the object 54// (which in turn may add those pointers to the work queue). 55// 56// e. Because GC work is spread across local caches, GC uses a 57// distributed termination algorithm to detect when there are no 58// more root marking jobs or grey objects (see gcMarkDone). At this 59// point, GC transitions to mark termination. 60// 61// 3. GC performs mark termination. 62// 63// a. Stop the world. 64// 65// b. Set gcphase to _GCmarktermination, and disable workers and 66// assists. 67// 68// c. Perform housekeeping like flushing mcaches. 69// 70// 4. GC performs the sweep phase. 71// 72// a. Prepare for the sweep phase by setting gcphase to _GCoff, 73// setting up sweep state and disabling the write barrier. 74// 75// b. Start the world. From this point on, newly allocated objects 76// are white, and allocating sweeps spans before use if necessary. 77// 78// c. GC does concurrent sweeping in the background and in response 79// to allocation. See description below. 80// 81// 5. When sufficient allocation has taken place, replay the sequence 82// starting with 1 above. See discussion of GC rate below. 83 84// Concurrent sweep. 85// 86// The sweep phase proceeds concurrently with normal program execution. 87// The heap is swept span-by-span both lazily (when a goroutine needs another span) 88// and concurrently in a background goroutine (this helps programs that are not CPU bound). 89// At the end of STW mark termination all spans are marked as "needs sweeping". 90// 91// The background sweeper goroutine simply sweeps spans one-by-one. 92// 93// To avoid requesting more OS memory while there are unswept spans, when a 94// goroutine needs another span, it first attempts to reclaim that much memory 95// by sweeping. When a goroutine needs to allocate a new small-object span, it 96// sweeps small-object spans for the same object size until it frees at least 97// one object. When a goroutine needs to allocate large-object span from heap, 98// it sweeps spans until it frees at least that many pages into heap. There is 99// one case where this may not suffice: if a goroutine sweeps and frees two 100// nonadjacent one-page spans to the heap, it will allocate a new two-page 101// span, but there can still be other one-page unswept spans which could be 102// combined into a two-page span. 103// 104// It's critical to ensure that no operations proceed on unswept spans (that would corrupt 105// mark bits in GC bitmap). During GC all mcaches are flushed into the central cache, 106// so they are empty. When a goroutine grabs a new span into mcache, it sweeps it. 107// When a goroutine explicitly frees an object or sets a finalizer, it ensures that 108// the span is swept (either by sweeping it, or by waiting for the concurrent sweep to finish). 109// The finalizer goroutine is kicked off only when all spans are swept. 110// When the next GC starts, it sweeps all not-yet-swept spans (if any). 111 112// GC rate. 113// Next GC is after we've allocated an extra amount of memory proportional to 114// the amount already in use. The proportion is controlled by GOGC environment variable 115// (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M 116// (this mark is tracked in next_gc variable). This keeps the GC cost in linear 117// proportion to the allocation cost. Adjusting GOGC just changes the linear constant 118// (and also the amount of extra memory used). 119 120// Oblets 121// 122// In order to prevent long pauses while scanning large objects and to 123// improve parallelism, the garbage collector breaks up scan jobs for 124// objects larger than maxObletBytes into "oblets" of at most 125// maxObletBytes. When scanning encounters the beginning of a large 126// object, it scans only the first oblet and enqueues the remaining 127// oblets as new scan jobs. 128 129package runtime 130 131import ( 132 "internal/cpu" 133 "runtime/internal/atomic" 134 "unsafe" 135) 136 137const ( 138 _DebugGC = 0 139 _ConcurrentSweep = true 140 _FinBlockSize = 4 * 1024 141 142 // debugScanConservative enables debug logging for stack 143 // frames that are scanned conservatively. 144 debugScanConservative = false 145 146 // sweepMinHeapDistance is a lower bound on the heap distance 147 // (in bytes) reserved for concurrent sweeping between GC 148 // cycles. 149 sweepMinHeapDistance = 1024 * 1024 150) 151 152// heapminimum is the minimum heap size at which to trigger GC. 153// For small heaps, this overrides the usual GOGC*live set rule. 154// 155// When there is a very small live set but a lot of allocation, simply 156// collecting when the heap reaches GOGC*live results in many GC 157// cycles and high total per-GC overhead. This minimum amortizes this 158// per-GC overhead while keeping the heap reasonably small. 159// 160// During initialization this is set to 4MB*GOGC/100. In the case of 161// GOGC==0, this will set heapminimum to 0, resulting in constant 162// collection even when the heap size is small, which is useful for 163// debugging. 164var heapminimum uint64 = defaultHeapMinimum 165 166// defaultHeapMinimum is the value of heapminimum for GOGC==100. 167const defaultHeapMinimum = 4 << 20 168 169// Initialized from $GOGC. GOGC=off means no GC. 170var gcpercent int32 171 172func gcinit() { 173 if unsafe.Sizeof(workbuf{}) != _WorkbufSize { 174 throw("size of Workbuf is suboptimal") 175 } 176 177 // No sweep on the first cycle. 178 mheap_.sweepdone = 1 179 180 // Set a reasonable initial GC trigger. 181 memstats.triggerRatio = 7 / 8.0 182 183 // Fake a heap_marked value so it looks like a trigger at 184 // heapminimum is the appropriate growth from heap_marked. 185 // This will go into computing the initial GC goal. 186 memstats.heap_marked = uint64(float64(heapminimum) / (1 + memstats.triggerRatio)) 187 188 // Set gcpercent from the environment. This will also compute 189 // and set the GC trigger and goal. 190 _ = setGCPercent(readgogc()) 191 192 work.startSema = 1 193 work.markDoneSema = 1 194} 195 196func readgogc() int32 { 197 p := gogetenv("GOGC") 198 if p == "off" { 199 return -1 200 } 201 if n, ok := atoi32(p); ok { 202 return n 203 } 204 return 100 205} 206 207// gcenable is called after the bulk of the runtime initialization, 208// just before we're about to start letting user code run. 209// It kicks off the background sweeper goroutine, the background 210// scavenger goroutine, and enables GC. 211func gcenable() { 212 // Kick off sweeping and scavenging. 213 c := make(chan int, 2) 214 go bgsweep(c) 215 go bgscavenge(c) 216 <-c 217 <-c 218 memstats.enablegc = true // now that runtime is initialized, GC is okay 219} 220 221//go:linkname setGCPercent runtime/debug.setGCPercent 222func setGCPercent(in int32) (out int32) { 223 // Run on the system stack since we grab the heap lock. 224 systemstack(func() { 225 lock(&mheap_.lock) 226 out = gcpercent 227 if in < 0 { 228 in = -1 229 } 230 gcpercent = in 231 heapminimum = defaultHeapMinimum * uint64(gcpercent) / 100 232 // Update pacing in response to gcpercent change. 233 gcSetTriggerRatio(memstats.triggerRatio) 234 unlock(&mheap_.lock) 235 }) 236 // Pacing changed, so the scavenger should be awoken. 237 wakeScavenger() 238 239 // If we just disabled GC, wait for any concurrent GC mark to 240 // finish so we always return with no GC running. 241 if in < 0 { 242 gcWaitOnMark(atomic.Load(&work.cycles)) 243 } 244 245 return out 246} 247 248// Garbage collector phase. 249// Indicates to write barrier and synchronization task to perform. 250var gcphase uint32 251 252// The compiler knows about this variable. 253// If you change it, you must change builtin/runtime.go, too. 254// If you change the first four bytes, you must also change the write 255// barrier insertion code. 256var writeBarrier struct { 257 enabled bool // compiler emits a check of this before calling write barrier 258 pad [3]byte // compiler uses 32-bit load for "enabled" field 259 needed bool // whether we need a write barrier for current GC phase 260 cgo bool // whether we need a write barrier for a cgo check 261 alignme uint64 // guarantee alignment so that compiler can use a 32 or 64-bit load 262} 263 264// gcBlackenEnabled is 1 if mutator assists and background mark 265// workers are allowed to blacken objects. This must only be set when 266// gcphase == _GCmark. 267var gcBlackenEnabled uint32 268 269const ( 270 _GCoff = iota // GC not running; sweeping in background, write barrier disabled 271 _GCmark // GC marking roots and workbufs: allocate black, write barrier ENABLED 272 _GCmarktermination // GC mark termination: allocate black, P's help GC, write barrier ENABLED 273) 274 275//go:nosplit 276func setGCPhase(x uint32) { 277 atomic.Store(&gcphase, x) 278 writeBarrier.needed = gcphase == _GCmark || gcphase == _GCmarktermination 279 writeBarrier.enabled = writeBarrier.needed || writeBarrier.cgo 280} 281 282// gcMarkWorkerMode represents the mode that a concurrent mark worker 283// should operate in. 284// 285// Concurrent marking happens through four different mechanisms. One 286// is mutator assists, which happen in response to allocations and are 287// not scheduled. The other three are variations in the per-P mark 288// workers and are distinguished by gcMarkWorkerMode. 289type gcMarkWorkerMode int 290 291const ( 292 // gcMarkWorkerDedicatedMode indicates that the P of a mark 293 // worker is dedicated to running that mark worker. The mark 294 // worker should run without preemption. 295 gcMarkWorkerDedicatedMode gcMarkWorkerMode = iota 296 297 // gcMarkWorkerFractionalMode indicates that a P is currently 298 // running the "fractional" mark worker. The fractional worker 299 // is necessary when GOMAXPROCS*gcBackgroundUtilization is not 300 // an integer. The fractional worker should run until it is 301 // preempted and will be scheduled to pick up the fractional 302 // part of GOMAXPROCS*gcBackgroundUtilization. 303 gcMarkWorkerFractionalMode 304 305 // gcMarkWorkerIdleMode indicates that a P is running the mark 306 // worker because it has nothing else to do. The idle worker 307 // should run until it is preempted and account its time 308 // against gcController.idleMarkTime. 309 gcMarkWorkerIdleMode 310) 311 312// gcMarkWorkerModeStrings are the strings labels of gcMarkWorkerModes 313// to use in execution traces. 314var gcMarkWorkerModeStrings = [...]string{ 315 "GC (dedicated)", 316 "GC (fractional)", 317 "GC (idle)", 318} 319 320// gcController implements the GC pacing controller that determines 321// when to trigger concurrent garbage collection and how much marking 322// work to do in mutator assists and background marking. 323// 324// It uses a feedback control algorithm to adjust the memstats.gc_trigger 325// trigger based on the heap growth and GC CPU utilization each cycle. 326// This algorithm optimizes for heap growth to match GOGC and for CPU 327// utilization between assist and background marking to be 25% of 328// GOMAXPROCS. The high-level design of this algorithm is documented 329// at https://golang.org/s/go15gcpacing. 330// 331// All fields of gcController are used only during a single mark 332// cycle. 333var gcController gcControllerState 334 335type gcControllerState struct { 336 // scanWork is the total scan work performed this cycle. This 337 // is updated atomically during the cycle. Updates occur in 338 // bounded batches, since it is both written and read 339 // throughout the cycle. At the end of the cycle, this is how 340 // much of the retained heap is scannable. 341 // 342 // Currently this is the bytes of heap scanned. For most uses, 343 // this is an opaque unit of work, but for estimation the 344 // definition is important. 345 scanWork int64 346 347 // bgScanCredit is the scan work credit accumulated by the 348 // concurrent background scan. This credit is accumulated by 349 // the background scan and stolen by mutator assists. This is 350 // updated atomically. Updates occur in bounded batches, since 351 // it is both written and read throughout the cycle. 352 bgScanCredit int64 353 354 // assistTime is the nanoseconds spent in mutator assists 355 // during this cycle. This is updated atomically. Updates 356 // occur in bounded batches, since it is both written and read 357 // throughout the cycle. 358 assistTime int64 359 360 // dedicatedMarkTime is the nanoseconds spent in dedicated 361 // mark workers during this cycle. This is updated atomically 362 // at the end of the concurrent mark phase. 363 dedicatedMarkTime int64 364 365 // fractionalMarkTime is the nanoseconds spent in the 366 // fractional mark worker during this cycle. This is updated 367 // atomically throughout the cycle and will be up-to-date if 368 // the fractional mark worker is not currently running. 369 fractionalMarkTime int64 370 371 // idleMarkTime is the nanoseconds spent in idle marking 372 // during this cycle. This is updated atomically throughout 373 // the cycle. 374 idleMarkTime int64 375 376 // markStartTime is the absolute start time in nanoseconds 377 // that assists and background mark workers started. 378 markStartTime int64 379 380 // dedicatedMarkWorkersNeeded is the number of dedicated mark 381 // workers that need to be started. This is computed at the 382 // beginning of each cycle and decremented atomically as 383 // dedicated mark workers get started. 384 dedicatedMarkWorkersNeeded int64 385 386 // assistWorkPerByte is the ratio of scan work to allocated 387 // bytes that should be performed by mutator assists. This is 388 // computed at the beginning of each cycle and updated every 389 // time heap_scan is updated. 390 assistWorkPerByte float64 391 392 // assistBytesPerWork is 1/assistWorkPerByte. 393 assistBytesPerWork float64 394 395 // fractionalUtilizationGoal is the fraction of wall clock 396 // time that should be spent in the fractional mark worker on 397 // each P that isn't running a dedicated worker. 398 // 399 // For example, if the utilization goal is 25% and there are 400 // no dedicated workers, this will be 0.25. If the goal is 401 // 25%, there is one dedicated worker, and GOMAXPROCS is 5, 402 // this will be 0.05 to make up the missing 5%. 403 // 404 // If this is zero, no fractional workers are needed. 405 fractionalUtilizationGoal float64 406 407 _ cpu.CacheLinePad 408} 409 410// startCycle resets the GC controller's state and computes estimates 411// for a new GC cycle. The caller must hold worldsema. 412func (c *gcControllerState) startCycle() { 413 c.scanWork = 0 414 c.bgScanCredit = 0 415 c.assistTime = 0 416 c.dedicatedMarkTime = 0 417 c.fractionalMarkTime = 0 418 c.idleMarkTime = 0 419 420 // Ensure that the heap goal is at least a little larger than 421 // the current live heap size. This may not be the case if GC 422 // start is delayed or if the allocation that pushed heap_live 423 // over gc_trigger is large or if the trigger is really close to 424 // GOGC. Assist is proportional to this distance, so enforce a 425 // minimum distance, even if it means going over the GOGC goal 426 // by a tiny bit. 427 if memstats.next_gc < memstats.heap_live+1024*1024 { 428 memstats.next_gc = memstats.heap_live + 1024*1024 429 } 430 431 // Compute the background mark utilization goal. In general, 432 // this may not come out exactly. We round the number of 433 // dedicated workers so that the utilization is closest to 434 // 25%. For small GOMAXPROCS, this would introduce too much 435 // error, so we add fractional workers in that case. 436 totalUtilizationGoal := float64(gomaxprocs) * gcBackgroundUtilization 437 c.dedicatedMarkWorkersNeeded = int64(totalUtilizationGoal + 0.5) 438 utilError := float64(c.dedicatedMarkWorkersNeeded)/totalUtilizationGoal - 1 439 const maxUtilError = 0.3 440 if utilError < -maxUtilError || utilError > maxUtilError { 441 // Rounding put us more than 30% off our goal. With 442 // gcBackgroundUtilization of 25%, this happens for 443 // GOMAXPROCS<=3 or GOMAXPROCS=6. Enable fractional 444 // workers to compensate. 445 if float64(c.dedicatedMarkWorkersNeeded) > totalUtilizationGoal { 446 // Too many dedicated workers. 447 c.dedicatedMarkWorkersNeeded-- 448 } 449 c.fractionalUtilizationGoal = (totalUtilizationGoal - float64(c.dedicatedMarkWorkersNeeded)) / float64(gomaxprocs) 450 } else { 451 c.fractionalUtilizationGoal = 0 452 } 453 454 // In STW mode, we just want dedicated workers. 455 if debug.gcstoptheworld > 0 { 456 c.dedicatedMarkWorkersNeeded = int64(gomaxprocs) 457 c.fractionalUtilizationGoal = 0 458 } 459 460 // Clear per-P state 461 for _, p := range allp { 462 p.gcAssistTime = 0 463 p.gcFractionalMarkTime = 0 464 } 465 466 // Compute initial values for controls that are updated 467 // throughout the cycle. 468 c.revise() 469 470 if debug.gcpacertrace > 0 { 471 print("pacer: assist ratio=", c.assistWorkPerByte, 472 " (scan ", memstats.heap_scan>>20, " MB in ", 473 work.initialHeapLive>>20, "->", 474 memstats.next_gc>>20, " MB)", 475 " workers=", c.dedicatedMarkWorkersNeeded, 476 "+", c.fractionalUtilizationGoal, "\n") 477 } 478} 479 480// revise updates the assist ratio during the GC cycle to account for 481// improved estimates. This should be called either under STW or 482// whenever memstats.heap_scan, memstats.heap_live, or 483// memstats.next_gc is updated (with mheap_.lock held). 484// 485// It should only be called when gcBlackenEnabled != 0 (because this 486// is when assists are enabled and the necessary statistics are 487// available). 488func (c *gcControllerState) revise() { 489 gcpercent := gcpercent 490 if gcpercent < 0 { 491 // If GC is disabled but we're running a forced GC, 492 // act like GOGC is huge for the below calculations. 493 gcpercent = 100000 494 } 495 live := atomic.Load64(&memstats.heap_live) 496 497 // Assume we're under the soft goal. Pace GC to complete at 498 // next_gc assuming the heap is in steady-state. 499 heapGoal := int64(memstats.next_gc) 500 501 // Compute the expected scan work remaining. 502 // 503 // This is estimated based on the expected 504 // steady-state scannable heap. For example, with 505 // GOGC=100, only half of the scannable heap is 506 // expected to be live, so that's what we target. 507 // 508 // (This is a float calculation to avoid overflowing on 509 // 100*heap_scan.) 510 scanWorkExpected := int64(float64(memstats.heap_scan) * 100 / float64(100+gcpercent)) 511 512 if live > memstats.next_gc || c.scanWork > scanWorkExpected { 513 // We're past the soft goal, or we've already done more scan 514 // work than we expected. Pace GC so that in the worst case it 515 // will complete by the hard goal. 516 const maxOvershoot = 1.1 517 heapGoal = int64(float64(memstats.next_gc) * maxOvershoot) 518 519 // Compute the upper bound on the scan work remaining. 520 scanWorkExpected = int64(memstats.heap_scan) 521 } 522 523 // Compute the remaining scan work estimate. 524 // 525 // Note that we currently count allocations during GC as both 526 // scannable heap (heap_scan) and scan work completed 527 // (scanWork), so allocation will change this difference 528 // slowly in the soft regime and not at all in the hard 529 // regime. 530 scanWorkRemaining := scanWorkExpected - c.scanWork 531 if scanWorkRemaining < 1000 { 532 // We set a somewhat arbitrary lower bound on 533 // remaining scan work since if we aim a little high, 534 // we can miss by a little. 535 // 536 // We *do* need to enforce that this is at least 1, 537 // since marking is racy and double-scanning objects 538 // may legitimately make the remaining scan work 539 // negative, even in the hard goal regime. 540 scanWorkRemaining = 1000 541 } 542 543 // Compute the heap distance remaining. 544 heapRemaining := heapGoal - int64(live) 545 if heapRemaining <= 0 { 546 // This shouldn't happen, but if it does, avoid 547 // dividing by zero or setting the assist negative. 548 heapRemaining = 1 549 } 550 551 // Compute the mutator assist ratio so by the time the mutator 552 // allocates the remaining heap bytes up to next_gc, it will 553 // have done (or stolen) the remaining amount of scan work. 554 c.assistWorkPerByte = float64(scanWorkRemaining) / float64(heapRemaining) 555 c.assistBytesPerWork = float64(heapRemaining) / float64(scanWorkRemaining) 556} 557 558// endCycle computes the trigger ratio for the next cycle. 559func (c *gcControllerState) endCycle() float64 { 560 if work.userForced { 561 // Forced GC means this cycle didn't start at the 562 // trigger, so where it finished isn't good 563 // information about how to adjust the trigger. 564 // Just leave it where it is. 565 return memstats.triggerRatio 566 } 567 568 // Proportional response gain for the trigger controller. Must 569 // be in [0, 1]. Lower values smooth out transient effects but 570 // take longer to respond to phase changes. Higher values 571 // react to phase changes quickly, but are more affected by 572 // transient changes. Values near 1 may be unstable. 573 const triggerGain = 0.5 574 575 // Compute next cycle trigger ratio. First, this computes the 576 // "error" for this cycle; that is, how far off the trigger 577 // was from what it should have been, accounting for both heap 578 // growth and GC CPU utilization. We compute the actual heap 579 // growth during this cycle and scale that by how far off from 580 // the goal CPU utilization we were (to estimate the heap 581 // growth if we had the desired CPU utilization). The 582 // difference between this estimate and the GOGC-based goal 583 // heap growth is the error. 584 goalGrowthRatio := gcEffectiveGrowthRatio() 585 actualGrowthRatio := float64(memstats.heap_live)/float64(memstats.heap_marked) - 1 586 assistDuration := nanotime() - c.markStartTime 587 588 // Assume background mark hit its utilization goal. 589 utilization := gcBackgroundUtilization 590 // Add assist utilization; avoid divide by zero. 591 if assistDuration > 0 { 592 utilization += float64(c.assistTime) / float64(assistDuration*int64(gomaxprocs)) 593 } 594 595 triggerError := goalGrowthRatio - memstats.triggerRatio - utilization/gcGoalUtilization*(actualGrowthRatio-memstats.triggerRatio) 596 597 // Finally, we adjust the trigger for next time by this error, 598 // damped by the proportional gain. 599 triggerRatio := memstats.triggerRatio + triggerGain*triggerError 600 601 if debug.gcpacertrace > 0 { 602 // Print controller state in terms of the design 603 // document. 604 H_m_prev := memstats.heap_marked 605 h_t := memstats.triggerRatio 606 H_T := memstats.gc_trigger 607 h_a := actualGrowthRatio 608 H_a := memstats.heap_live 609 h_g := goalGrowthRatio 610 H_g := int64(float64(H_m_prev) * (1 + h_g)) 611 u_a := utilization 612 u_g := gcGoalUtilization 613 W_a := c.scanWork 614 print("pacer: H_m_prev=", H_m_prev, 615 " h_t=", h_t, " H_T=", H_T, 616 " h_a=", h_a, " H_a=", H_a, 617 " h_g=", h_g, " H_g=", H_g, 618 " u_a=", u_a, " u_g=", u_g, 619 " W_a=", W_a, 620 " goalΔ=", goalGrowthRatio-h_t, 621 " actualΔ=", h_a-h_t, 622 " u_a/u_g=", u_a/u_g, 623 "\n") 624 } 625 626 return triggerRatio 627} 628 629// enlistWorker encourages another dedicated mark worker to start on 630// another P if there are spare worker slots. It is used by putfull 631// when more work is made available. 632// 633//go:nowritebarrier 634func (c *gcControllerState) enlistWorker() { 635 // If there are idle Ps, wake one so it will run an idle worker. 636 // NOTE: This is suspected of causing deadlocks. See golang.org/issue/19112. 637 // 638 // if atomic.Load(&sched.npidle) != 0 && atomic.Load(&sched.nmspinning) == 0 { 639 // wakep() 640 // return 641 // } 642 643 // There are no idle Ps. If we need more dedicated workers, 644 // try to preempt a running P so it will switch to a worker. 645 if c.dedicatedMarkWorkersNeeded <= 0 { 646 return 647 } 648 // Pick a random other P to preempt. 649 if gomaxprocs <= 1 { 650 return 651 } 652 gp := getg() 653 if gp == nil || gp.m == nil || gp.m.p == 0 { 654 return 655 } 656 myID := gp.m.p.ptr().id 657 for tries := 0; tries < 5; tries++ { 658 id := int32(fastrandn(uint32(gomaxprocs - 1))) 659 if id >= myID { 660 id++ 661 } 662 p := allp[id] 663 if p.status != _Prunning { 664 continue 665 } 666 if preemptone(p) { 667 return 668 } 669 } 670} 671 672// findRunnableGCWorker returns the background mark worker for _p_ if it 673// should be run. This must only be called when gcBlackenEnabled != 0. 674func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g { 675 if gcBlackenEnabled == 0 { 676 throw("gcControllerState.findRunnable: blackening not enabled") 677 } 678 if _p_.gcBgMarkWorker == 0 { 679 // The mark worker associated with this P is blocked 680 // performing a mark transition. We can't run it 681 // because it may be on some other run or wait queue. 682 return nil 683 } 684 685 if !gcMarkWorkAvailable(_p_) { 686 // No work to be done right now. This can happen at 687 // the end of the mark phase when there are still 688 // assists tapering off. Don't bother running a worker 689 // now because it'll just return immediately. 690 return nil 691 } 692 693 decIfPositive := func(ptr *int64) bool { 694 if *ptr > 0 { 695 if atomic.Xaddint64(ptr, -1) >= 0 { 696 return true 697 } 698 // We lost a race 699 atomic.Xaddint64(ptr, +1) 700 } 701 return false 702 } 703 704 if decIfPositive(&c.dedicatedMarkWorkersNeeded) { 705 // This P is now dedicated to marking until the end of 706 // the concurrent mark phase. 707 _p_.gcMarkWorkerMode = gcMarkWorkerDedicatedMode 708 } else if c.fractionalUtilizationGoal == 0 { 709 // No need for fractional workers. 710 return nil 711 } else { 712 // Is this P behind on the fractional utilization 713 // goal? 714 // 715 // This should be kept in sync with pollFractionalWorkerExit. 716 delta := nanotime() - gcController.markStartTime 717 if delta > 0 && float64(_p_.gcFractionalMarkTime)/float64(delta) > c.fractionalUtilizationGoal { 718 // Nope. No need to run a fractional worker. 719 return nil 720 } 721 // Run a fractional worker. 722 _p_.gcMarkWorkerMode = gcMarkWorkerFractionalMode 723 } 724 725 // Run the background mark worker 726 gp := _p_.gcBgMarkWorker.ptr() 727 casgstatus(gp, _Gwaiting, _Grunnable) 728 if trace.enabled { 729 traceGoUnpark(gp, 0) 730 } 731 return gp 732} 733 734// pollFractionalWorkerExit reports whether a fractional mark worker 735// should self-preempt. It assumes it is called from the fractional 736// worker. 737func pollFractionalWorkerExit() bool { 738 // This should be kept in sync with the fractional worker 739 // scheduler logic in findRunnableGCWorker. 740 now := nanotime() 741 delta := now - gcController.markStartTime 742 if delta <= 0 { 743 return true 744 } 745 p := getg().m.p.ptr() 746 selfTime := p.gcFractionalMarkTime + (now - p.gcMarkWorkerStartTime) 747 // Add some slack to the utilization goal so that the 748 // fractional worker isn't behind again the instant it exits. 749 return float64(selfTime)/float64(delta) > 1.2*gcController.fractionalUtilizationGoal 750} 751 752// gcSetTriggerRatio sets the trigger ratio and updates everything 753// derived from it: the absolute trigger, the heap goal, mark pacing, 754// and sweep pacing. 755// 756// This can be called any time. If GC is the in the middle of a 757// concurrent phase, it will adjust the pacing of that phase. 758// 759// This depends on gcpercent, memstats.heap_marked, and 760// memstats.heap_live. These must be up to date. 761// 762// mheap_.lock must be held or the world must be stopped. 763func gcSetTriggerRatio(triggerRatio float64) { 764 // Compute the next GC goal, which is when the allocated heap 765 // has grown by GOGC/100 over the heap marked by the last 766 // cycle. 767 goal := ^uint64(0) 768 if gcpercent >= 0 { 769 goal = memstats.heap_marked + memstats.heap_marked*uint64(gcpercent)/100 770 } 771 772 // If we let triggerRatio go too low, then if the application 773 // is allocating very rapidly we might end up in a situation 774 // where we're allocating black during a nearly always-on GC. 775 // The result of this is a growing heap and ultimately an 776 // increase in RSS. By capping us at a point >0, we're essentially 777 // saying that we're OK using more CPU during the GC to prevent 778 // this growth in RSS. 779 // 780 // The current constant was chosen empirically: given a sufficiently 781 // fast/scalable allocator with 48 Ps that could drive the trigger ratio 782 // to <0.05, this constant causes applications to retain the same peak 783 // RSS compared to not having this allocator. 784 const minTriggerRatio = 0.6 785 786 // Set the trigger ratio, capped to reasonable bounds. 787 if triggerRatio < minTriggerRatio { 788 // This can happen if the mutator is allocating very 789 // quickly or the GC is scanning very slowly. 790 triggerRatio = minTriggerRatio 791 } else if gcpercent >= 0 { 792 // Ensure there's always a little margin so that the 793 // mutator assist ratio isn't infinity. 794 maxTriggerRatio := 0.95 * float64(gcpercent) / 100 795 if triggerRatio > maxTriggerRatio { 796 triggerRatio = maxTriggerRatio 797 } 798 } 799 memstats.triggerRatio = triggerRatio 800 801 // Compute the absolute GC trigger from the trigger ratio. 802 // 803 // We trigger the next GC cycle when the allocated heap has 804 // grown by the trigger ratio over the marked heap size. 805 trigger := ^uint64(0) 806 if gcpercent >= 0 { 807 trigger = uint64(float64(memstats.heap_marked) * (1 + triggerRatio)) 808 // Don't trigger below the minimum heap size. 809 minTrigger := heapminimum 810 if !isSweepDone() { 811 // Concurrent sweep happens in the heap growth 812 // from heap_live to gc_trigger, so ensure 813 // that concurrent sweep has some heap growth 814 // in which to perform sweeping before we 815 // start the next GC cycle. 816 sweepMin := atomic.Load64(&memstats.heap_live) + sweepMinHeapDistance 817 if sweepMin > minTrigger { 818 minTrigger = sweepMin 819 } 820 } 821 if trigger < minTrigger { 822 trigger = minTrigger 823 } 824 if int64(trigger) < 0 { 825 print("runtime: next_gc=", memstats.next_gc, " heap_marked=", memstats.heap_marked, " heap_live=", memstats.heap_live, " initialHeapLive=", work.initialHeapLive, "triggerRatio=", triggerRatio, " minTrigger=", minTrigger, "\n") 826 throw("gc_trigger underflow") 827 } 828 if trigger > goal { 829 // The trigger ratio is always less than GOGC/100, but 830 // other bounds on the trigger may have raised it. 831 // Push up the goal, too. 832 goal = trigger 833 } 834 } 835 836 // Commit to the trigger and goal. 837 memstats.gc_trigger = trigger 838 memstats.next_gc = goal 839 if trace.enabled { 840 traceNextGC() 841 } 842 843 // Update mark pacing. 844 if gcphase != _GCoff { 845 gcController.revise() 846 } 847 848 // Update sweep pacing. 849 if isSweepDone() { 850 mheap_.sweepPagesPerByte = 0 851 } else { 852 // Concurrent sweep needs to sweep all of the in-use 853 // pages by the time the allocated heap reaches the GC 854 // trigger. Compute the ratio of in-use pages to sweep 855 // per byte allocated, accounting for the fact that 856 // some might already be swept. 857 heapLiveBasis := atomic.Load64(&memstats.heap_live) 858 heapDistance := int64(trigger) - int64(heapLiveBasis) 859 // Add a little margin so rounding errors and 860 // concurrent sweep are less likely to leave pages 861 // unswept when GC starts. 862 heapDistance -= 1024 * 1024 863 if heapDistance < _PageSize { 864 // Avoid setting the sweep ratio extremely high 865 heapDistance = _PageSize 866 } 867 pagesSwept := atomic.Load64(&mheap_.pagesSwept) 868 pagesInUse := atomic.Load64(&mheap_.pagesInUse) 869 sweepDistancePages := int64(pagesInUse) - int64(pagesSwept) 870 if sweepDistancePages <= 0 { 871 mheap_.sweepPagesPerByte = 0 872 } else { 873 mheap_.sweepPagesPerByte = float64(sweepDistancePages) / float64(heapDistance) 874 mheap_.sweepHeapLiveBasis = heapLiveBasis 875 // Write pagesSweptBasis last, since this 876 // signals concurrent sweeps to recompute 877 // their debt. 878 atomic.Store64(&mheap_.pagesSweptBasis, pagesSwept) 879 } 880 } 881 882 gcPaceScavenger() 883} 884 885// gcEffectiveGrowthRatio returns the current effective heap growth 886// ratio (GOGC/100) based on heap_marked from the previous GC and 887// next_gc for the current GC. 888// 889// This may differ from gcpercent/100 because of various upper and 890// lower bounds on gcpercent. For example, if the heap is smaller than 891// heapminimum, this can be higher than gcpercent/100. 892// 893// mheap_.lock must be held or the world must be stopped. 894func gcEffectiveGrowthRatio() float64 { 895 egogc := float64(memstats.next_gc-memstats.heap_marked) / float64(memstats.heap_marked) 896 if egogc < 0 { 897 // Shouldn't happen, but just in case. 898 egogc = 0 899 } 900 return egogc 901} 902 903// gcGoalUtilization is the goal CPU utilization for 904// marking as a fraction of GOMAXPROCS. 905const gcGoalUtilization = 0.30 906 907// gcBackgroundUtilization is the fixed CPU utilization for background 908// marking. It must be <= gcGoalUtilization. The difference between 909// gcGoalUtilization and gcBackgroundUtilization will be made up by 910// mark assists. The scheduler will aim to use within 50% of this 911// goal. 912// 913// Setting this to < gcGoalUtilization avoids saturating the trigger 914// feedback controller when there are no assists, which allows it to 915// better control CPU and heap growth. However, the larger the gap, 916// the more mutator assists are expected to happen, which impact 917// mutator latency. 918const gcBackgroundUtilization = 0.25 919 920// gcCreditSlack is the amount of scan work credit that can 921// accumulate locally before updating gcController.scanWork and, 922// optionally, gcController.bgScanCredit. Lower values give a more 923// accurate assist ratio and make it more likely that assists will 924// successfully steal background credit. Higher values reduce memory 925// contention. 926const gcCreditSlack = 2000 927 928// gcAssistTimeSlack is the nanoseconds of mutator assist time that 929// can accumulate on a P before updating gcController.assistTime. 930const gcAssistTimeSlack = 5000 931 932// gcOverAssistWork determines how many extra units of scan work a GC 933// assist does when an assist happens. This amortizes the cost of an 934// assist by pre-paying for this many bytes of future allocations. 935const gcOverAssistWork = 64 << 10 936 937var work struct { 938 full lfstack // lock-free list of full blocks workbuf 939 empty lfstack // lock-free list of empty blocks workbuf 940 pad0 cpu.CacheLinePad // prevents false-sharing between full/empty and nproc/nwait 941 942 wbufSpans struct { 943 lock mutex 944 // free is a list of spans dedicated to workbufs, but 945 // that don't currently contain any workbufs. 946 free mSpanList 947 // busy is a list of all spans containing workbufs on 948 // one of the workbuf lists. 949 busy mSpanList 950 } 951 952 // Restore 64-bit alignment on 32-bit. 953 _ uint32 954 955 // bytesMarked is the number of bytes marked this cycle. This 956 // includes bytes blackened in scanned objects, noscan objects 957 // that go straight to black, and permagrey objects scanned by 958 // markroot during the concurrent scan phase. This is updated 959 // atomically during the cycle. Updates may be batched 960 // arbitrarily, since the value is only read at the end of the 961 // cycle. 962 // 963 // Because of benign races during marking, this number may not 964 // be the exact number of marked bytes, but it should be very 965 // close. 966 // 967 // Put this field here because it needs 64-bit atomic access 968 // (and thus 8-byte alignment even on 32-bit architectures). 969 bytesMarked uint64 970 971 markrootNext uint32 // next markroot job 972 markrootJobs uint32 // number of markroot jobs 973 974 nproc uint32 975 tstart int64 976 nwait uint32 977 ndone uint32 978 979 // Number of roots of various root types. Set by gcMarkRootPrepare. 980 nFlushCacheRoots int 981 nDataRoots, nBSSRoots, nSpanRoots, nStackRoots int 982 983 // Each type of GC state transition is protected by a lock. 984 // Since multiple threads can simultaneously detect the state 985 // transition condition, any thread that detects a transition 986 // condition must acquire the appropriate transition lock, 987 // re-check the transition condition and return if it no 988 // longer holds or perform the transition if it does. 989 // Likewise, any transition must invalidate the transition 990 // condition before releasing the lock. This ensures that each 991 // transition is performed by exactly one thread and threads 992 // that need the transition to happen block until it has 993 // happened. 994 // 995 // startSema protects the transition from "off" to mark or 996 // mark termination. 997 startSema uint32 998 // markDoneSema protects transitions from mark to mark termination. 999 markDoneSema uint32 1000 1001 bgMarkReady note // signal background mark worker has started 1002 bgMarkDone uint32 // cas to 1 when at a background mark completion point 1003 // Background mark completion signaling 1004 1005 // mode is the concurrency mode of the current GC cycle. 1006 mode gcMode 1007 1008 // userForced indicates the current GC cycle was forced by an 1009 // explicit user call. 1010 userForced bool 1011 1012 // totaltime is the CPU nanoseconds spent in GC since the 1013 // program started if debug.gctrace > 0. 1014 totaltime int64 1015 1016 // initialHeapLive is the value of memstats.heap_live at the 1017 // beginning of this GC cycle. 1018 initialHeapLive uint64 1019 1020 // assistQueue is a queue of assists that are blocked because 1021 // there was neither enough credit to steal or enough work to 1022 // do. 1023 assistQueue struct { 1024 lock mutex 1025 q gQueue 1026 } 1027 1028 // sweepWaiters is a list of blocked goroutines to wake when 1029 // we transition from mark termination to sweep. 1030 sweepWaiters struct { 1031 lock mutex 1032 list gList 1033 } 1034 1035 // cycles is the number of completed GC cycles, where a GC 1036 // cycle is sweep termination, mark, mark termination, and 1037 // sweep. This differs from memstats.numgc, which is 1038 // incremented at mark termination. 1039 cycles uint32 1040 1041 // Timing/utilization stats for this cycle. 1042 stwprocs, maxprocs int32 1043 tSweepTerm, tMark, tMarkTerm, tEnd int64 // nanotime() of phase start 1044 1045 pauseNS int64 // total STW time this cycle 1046 pauseStart int64 // nanotime() of last STW 1047 1048 // debug.gctrace heap sizes for this cycle. 1049 heap0, heap1, heap2, heapGoal uint64 1050} 1051 1052// GC runs a garbage collection and blocks the caller until the 1053// garbage collection is complete. It may also block the entire 1054// program. 1055func GC() { 1056 // We consider a cycle to be: sweep termination, mark, mark 1057 // termination, and sweep. This function shouldn't return 1058 // until a full cycle has been completed, from beginning to 1059 // end. Hence, we always want to finish up the current cycle 1060 // and start a new one. That means: 1061 // 1062 // 1. In sweep termination, mark, or mark termination of cycle 1063 // N, wait until mark termination N completes and transitions 1064 // to sweep N. 1065 // 1066 // 2. In sweep N, help with sweep N. 1067 // 1068 // At this point we can begin a full cycle N+1. 1069 // 1070 // 3. Trigger cycle N+1 by starting sweep termination N+1. 1071 // 1072 // 4. Wait for mark termination N+1 to complete. 1073 // 1074 // 5. Help with sweep N+1 until it's done. 1075 // 1076 // This all has to be written to deal with the fact that the 1077 // GC may move ahead on its own. For example, when we block 1078 // until mark termination N, we may wake up in cycle N+2. 1079 1080 // Wait until the current sweep termination, mark, and mark 1081 // termination complete. 1082 n := atomic.Load(&work.cycles) 1083 gcWaitOnMark(n) 1084 1085 // We're now in sweep N or later. Trigger GC cycle N+1, which 1086 // will first finish sweep N if necessary and then enter sweep 1087 // termination N+1. 1088 gcStart(gcTrigger{kind: gcTriggerCycle, n: n + 1}) 1089 1090 // Wait for mark termination N+1 to complete. 1091 gcWaitOnMark(n + 1) 1092 1093 // Finish sweep N+1 before returning. We do this both to 1094 // complete the cycle and because runtime.GC() is often used 1095 // as part of tests and benchmarks to get the system into a 1096 // relatively stable and isolated state. 1097 for atomic.Load(&work.cycles) == n+1 && sweepone() != ^uintptr(0) { 1098 sweep.nbgsweep++ 1099 Gosched() 1100 } 1101 1102 // Callers may assume that the heap profile reflects the 1103 // just-completed cycle when this returns (historically this 1104 // happened because this was a STW GC), but right now the 1105 // profile still reflects mark termination N, not N+1. 1106 // 1107 // As soon as all of the sweep frees from cycle N+1 are done, 1108 // we can go ahead and publish the heap profile. 1109 // 1110 // First, wait for sweeping to finish. (We know there are no 1111 // more spans on the sweep queue, but we may be concurrently 1112 // sweeping spans, so we have to wait.) 1113 for atomic.Load(&work.cycles) == n+1 && atomic.Load(&mheap_.sweepers) != 0 { 1114 Gosched() 1115 } 1116 1117 // Now we're really done with sweeping, so we can publish the 1118 // stable heap profile. Only do this if we haven't already hit 1119 // another mark termination. 1120 mp := acquirem() 1121 cycle := atomic.Load(&work.cycles) 1122 if cycle == n+1 || (gcphase == _GCmark && cycle == n+2) { 1123 mProf_PostSweep() 1124 } 1125 releasem(mp) 1126} 1127 1128// gcWaitOnMark blocks until GC finishes the Nth mark phase. If GC has 1129// already completed this mark phase, it returns immediately. 1130func gcWaitOnMark(n uint32) { 1131 for { 1132 // Disable phase transitions. 1133 lock(&work.sweepWaiters.lock) 1134 nMarks := atomic.Load(&work.cycles) 1135 if gcphase != _GCmark { 1136 // We've already completed this cycle's mark. 1137 nMarks++ 1138 } 1139 if nMarks > n { 1140 // We're done. 1141 unlock(&work.sweepWaiters.lock) 1142 return 1143 } 1144 1145 // Wait until sweep termination, mark, and mark 1146 // termination of cycle N complete. 1147 work.sweepWaiters.list.push(getg()) 1148 goparkunlock(&work.sweepWaiters.lock, waitReasonWaitForGCCycle, traceEvGoBlock, 1) 1149 } 1150} 1151 1152// gcMode indicates how concurrent a GC cycle should be. 1153type gcMode int 1154 1155const ( 1156 gcBackgroundMode gcMode = iota // concurrent GC and sweep 1157 gcForceMode // stop-the-world GC now, concurrent sweep 1158 gcForceBlockMode // stop-the-world GC now and STW sweep (forced by user) 1159) 1160 1161// A gcTrigger is a predicate for starting a GC cycle. Specifically, 1162// it is an exit condition for the _GCoff phase. 1163type gcTrigger struct { 1164 kind gcTriggerKind 1165 now int64 // gcTriggerTime: current time 1166 n uint32 // gcTriggerCycle: cycle number to start 1167} 1168 1169type gcTriggerKind int 1170 1171const ( 1172 // gcTriggerHeap indicates that a cycle should be started when 1173 // the heap size reaches the trigger heap size computed by the 1174 // controller. 1175 gcTriggerHeap gcTriggerKind = iota 1176 1177 // gcTriggerTime indicates that a cycle should be started when 1178 // it's been more than forcegcperiod nanoseconds since the 1179 // previous GC cycle. 1180 gcTriggerTime 1181 1182 // gcTriggerCycle indicates that a cycle should be started if 1183 // we have not yet started cycle number gcTrigger.n (relative 1184 // to work.cycles). 1185 gcTriggerCycle 1186) 1187 1188// test reports whether the trigger condition is satisfied, meaning 1189// that the exit condition for the _GCoff phase has been met. The exit 1190// condition should be tested when allocating. 1191func (t gcTrigger) test() bool { 1192 if !memstats.enablegc || panicking != 0 || gcphase != _GCoff { 1193 return false 1194 } 1195 switch t.kind { 1196 case gcTriggerHeap: 1197 // Non-atomic access to heap_live for performance. If 1198 // we are going to trigger on this, this thread just 1199 // atomically wrote heap_live anyway and we'll see our 1200 // own write. 1201 return memstats.heap_live >= memstats.gc_trigger 1202 case gcTriggerTime: 1203 if gcpercent < 0 { 1204 return false 1205 } 1206 lastgc := int64(atomic.Load64(&memstats.last_gc_nanotime)) 1207 return lastgc != 0 && t.now-lastgc > forcegcperiod 1208 case gcTriggerCycle: 1209 // t.n > work.cycles, but accounting for wraparound. 1210 return int32(t.n-work.cycles) > 0 1211 } 1212 return true 1213} 1214 1215// gcStart starts the GC. It transitions from _GCoff to _GCmark (if 1216// debug.gcstoptheworld == 0) or performs all of GC (if 1217// debug.gcstoptheworld != 0). 1218// 1219// This may return without performing this transition in some cases, 1220// such as when called on a system stack or with locks held. 1221func gcStart(trigger gcTrigger) { 1222 // Since this is called from malloc and malloc is called in 1223 // the guts of a number of libraries that might be holding 1224 // locks, don't attempt to start GC in non-preemptible or 1225 // potentially unstable situations. 1226 mp := acquirem() 1227 if gp := getg(); gp == mp.g0 || mp.locks > 1 || mp.preemptoff != "" { 1228 releasem(mp) 1229 return 1230 } 1231 releasem(mp) 1232 mp = nil 1233 1234 // Pick up the remaining unswept/not being swept spans concurrently 1235 // 1236 // This shouldn't happen if we're being invoked in background 1237 // mode since proportional sweep should have just finished 1238 // sweeping everything, but rounding errors, etc, may leave a 1239 // few spans unswept. In forced mode, this is necessary since 1240 // GC can be forced at any point in the sweeping cycle. 1241 // 1242 // We check the transition condition continuously here in case 1243 // this G gets delayed in to the next GC cycle. 1244 for trigger.test() && sweepone() != ^uintptr(0) { 1245 sweep.nbgsweep++ 1246 } 1247 1248 // Perform GC initialization and the sweep termination 1249 // transition. 1250 semacquire(&work.startSema) 1251 // Re-check transition condition under transition lock. 1252 if !trigger.test() { 1253 semrelease(&work.startSema) 1254 return 1255 } 1256 1257 // For stats, check if this GC was forced by the user. 1258 work.userForced = trigger.kind == gcTriggerCycle 1259 1260 // In gcstoptheworld debug mode, upgrade the mode accordingly. 1261 // We do this after re-checking the transition condition so 1262 // that multiple goroutines that detect the heap trigger don't 1263 // start multiple STW GCs. 1264 mode := gcBackgroundMode 1265 if debug.gcstoptheworld == 1 { 1266 mode = gcForceMode 1267 } else if debug.gcstoptheworld == 2 { 1268 mode = gcForceBlockMode 1269 } 1270 1271 // Ok, we're doing it! Stop everybody else 1272 semacquire(&worldsema) 1273 1274 if trace.enabled { 1275 traceGCStart() 1276 } 1277 1278 // Check that all Ps have finished deferred mcache flushes. 1279 for _, p := range allp { 1280 if fg := atomic.Load(&p.mcache.flushGen); fg != mheap_.sweepgen { 1281 println("runtime: p", p.id, "flushGen", fg, "!= sweepgen", mheap_.sweepgen) 1282 throw("p mcache not flushed") 1283 } 1284 } 1285 1286 gcBgMarkStartWorkers() 1287 1288 systemstack(gcResetMarkState) 1289 1290 work.stwprocs, work.maxprocs = gomaxprocs, gomaxprocs 1291 if work.stwprocs > ncpu { 1292 // This is used to compute CPU time of the STW phases, 1293 // so it can't be more than ncpu, even if GOMAXPROCS is. 1294 work.stwprocs = ncpu 1295 } 1296 work.heap0 = atomic.Load64(&memstats.heap_live) 1297 work.pauseNS = 0 1298 work.mode = mode 1299 1300 now := nanotime() 1301 work.tSweepTerm = now 1302 work.pauseStart = now 1303 if trace.enabled { 1304 traceGCSTWStart(1) 1305 } 1306 systemstack(stopTheWorldWithSema) 1307 // Finish sweep before we start concurrent scan. 1308 systemstack(func() { 1309 finishsweep_m() 1310 }) 1311 // clearpools before we start the GC. If we wait they memory will not be 1312 // reclaimed until the next GC cycle. 1313 clearpools() 1314 1315 work.cycles++ 1316 1317 gcController.startCycle() 1318 work.heapGoal = memstats.next_gc 1319 1320 // In STW mode, disable scheduling of user Gs. This may also 1321 // disable scheduling of this goroutine, so it may block as 1322 // soon as we start the world again. 1323 if mode != gcBackgroundMode { 1324 schedEnableUser(false) 1325 } 1326 1327 // Enter concurrent mark phase and enable 1328 // write barriers. 1329 // 1330 // Because the world is stopped, all Ps will 1331 // observe that write barriers are enabled by 1332 // the time we start the world and begin 1333 // scanning. 1334 // 1335 // Write barriers must be enabled before assists are 1336 // enabled because they must be enabled before 1337 // any non-leaf heap objects are marked. Since 1338 // allocations are blocked until assists can 1339 // happen, we want enable assists as early as 1340 // possible. 1341 setGCPhase(_GCmark) 1342 1343 gcBgMarkPrepare() // Must happen before assist enable. 1344 gcMarkRootPrepare() 1345 1346 // Mark all active tinyalloc blocks. Since we're 1347 // allocating from these, they need to be black like 1348 // other allocations. The alternative is to blacken 1349 // the tiny block on every allocation from it, which 1350 // would slow down the tiny allocator. 1351 gcMarkTinyAllocs() 1352 1353 // At this point all Ps have enabled the write 1354 // barrier, thus maintaining the no white to 1355 // black invariant. Enable mutator assists to 1356 // put back-pressure on fast allocating 1357 // mutators. 1358 atomic.Store(&gcBlackenEnabled, 1) 1359 1360 // Assists and workers can start the moment we start 1361 // the world. 1362 gcController.markStartTime = now 1363 1364 // Concurrent mark. 1365 systemstack(func() { 1366 now = startTheWorldWithSema(trace.enabled) 1367 work.pauseNS += now - work.pauseStart 1368 work.tMark = now 1369 }) 1370 // In STW mode, we could block the instant systemstack 1371 // returns, so don't do anything important here. Make sure we 1372 // block rather than returning to user code. 1373 if mode != gcBackgroundMode { 1374 Gosched() 1375 } 1376 1377 semrelease(&work.startSema) 1378} 1379 1380// gcMarkDoneFlushed counts the number of P's with flushed work. 1381// 1382// Ideally this would be a captured local in gcMarkDone, but forEachP 1383// escapes its callback closure, so it can't capture anything. 1384// 1385// This is protected by markDoneSema. 1386var gcMarkDoneFlushed uint32 1387 1388// debugCachedWork enables extra checks for debugging premature mark 1389// termination. 1390// 1391// For debugging issue #27993. 1392const debugCachedWork = false 1393 1394// gcWorkPauseGen is for debugging the mark completion algorithm. 1395// gcWork put operations spin while gcWork.pauseGen == gcWorkPauseGen. 1396// Only used if debugCachedWork is true. 1397// 1398// For debugging issue #27993. 1399var gcWorkPauseGen uint32 = 1 1400 1401// gcMarkDone transitions the GC from mark to mark termination if all 1402// reachable objects have been marked (that is, there are no grey 1403// objects and can be no more in the future). Otherwise, it flushes 1404// all local work to the global queues where it can be discovered by 1405// other workers. 1406// 1407// This should be called when all local mark work has been drained and 1408// there are no remaining workers. Specifically, when 1409// 1410// work.nwait == work.nproc && !gcMarkWorkAvailable(p) 1411// 1412// The calling context must be preemptible. 1413// 1414// Flushing local work is important because idle Ps may have local 1415// work queued. This is the only way to make that work visible and 1416// drive GC to completion. 1417// 1418// It is explicitly okay to have write barriers in this function. If 1419// it does transition to mark termination, then all reachable objects 1420// have been marked, so the write barrier cannot shade any more 1421// objects. 1422func gcMarkDone() { 1423 // Ensure only one thread is running the ragged barrier at a 1424 // time. 1425 semacquire(&work.markDoneSema) 1426 1427top: 1428 // Re-check transition condition under transition lock. 1429 // 1430 // It's critical that this checks the global work queues are 1431 // empty before performing the ragged barrier. Otherwise, 1432 // there could be global work that a P could take after the P 1433 // has passed the ragged barrier. 1434 if !(gcphase == _GCmark && work.nwait == work.nproc && !gcMarkWorkAvailable(nil)) { 1435 semrelease(&work.markDoneSema) 1436 return 1437 } 1438 1439 // Flush all local buffers and collect flushedWork flags. 1440 gcMarkDoneFlushed = 0 1441 systemstack(func() { 1442 gp := getg().m.curg 1443 // Mark the user stack as preemptible so that it may be scanned. 1444 // Otherwise, our attempt to force all P's to a safepoint could 1445 // result in a deadlock as we attempt to preempt a worker that's 1446 // trying to preempt us (e.g. for a stack scan). 1447 casgstatus(gp, _Grunning, _Gwaiting) 1448 forEachP(func(_p_ *p) { 1449 // Flush the write barrier buffer, since this may add 1450 // work to the gcWork. 1451 wbBufFlush1(_p_) 1452 // For debugging, shrink the write barrier 1453 // buffer so it flushes immediately. 1454 // wbBuf.reset will keep it at this size as 1455 // long as throwOnGCWork is set. 1456 if debugCachedWork { 1457 b := &_p_.wbBuf 1458 b.end = uintptr(unsafe.Pointer(&b.buf[wbBufEntryPointers])) 1459 b.debugGen = gcWorkPauseGen 1460 } 1461 // Flush the gcWork, since this may create global work 1462 // and set the flushedWork flag. 1463 // 1464 // TODO(austin): Break up these workbufs to 1465 // better distribute work. 1466 _p_.gcw.dispose() 1467 // Collect the flushedWork flag. 1468 if _p_.gcw.flushedWork { 1469 atomic.Xadd(&gcMarkDoneFlushed, 1) 1470 _p_.gcw.flushedWork = false 1471 } else if debugCachedWork { 1472 // For debugging, freeze the gcWork 1473 // until we know whether we've reached 1474 // completion or not. If we think 1475 // we've reached completion, but 1476 // there's a paused gcWork, then 1477 // that's a bug. 1478 _p_.gcw.pauseGen = gcWorkPauseGen 1479 // Capture the G's stack. 1480 for i := range _p_.gcw.pauseStack { 1481 _p_.gcw.pauseStack[i] = 0 1482 } 1483 callers(1, _p_.gcw.pauseStack[:]) 1484 } 1485 }) 1486 casgstatus(gp, _Gwaiting, _Grunning) 1487 }) 1488 1489 if gcMarkDoneFlushed != 0 { 1490 if debugCachedWork { 1491 // Release paused gcWorks. 1492 atomic.Xadd(&gcWorkPauseGen, 1) 1493 } 1494 // More grey objects were discovered since the 1495 // previous termination check, so there may be more 1496 // work to do. Keep going. It's possible the 1497 // transition condition became true again during the 1498 // ragged barrier, so re-check it. 1499 goto top 1500 } 1501 1502 if debugCachedWork { 1503 throwOnGCWork = true 1504 // Release paused gcWorks. If there are any, they 1505 // should now observe throwOnGCWork and panic. 1506 atomic.Xadd(&gcWorkPauseGen, 1) 1507 } 1508 1509 // There was no global work, no local work, and no Ps 1510 // communicated work since we took markDoneSema. Therefore 1511 // there are no grey objects and no more objects can be 1512 // shaded. Transition to mark termination. 1513 now := nanotime() 1514 work.tMarkTerm = now 1515 work.pauseStart = now 1516 getg().m.preemptoff = "gcing" 1517 if trace.enabled { 1518 traceGCSTWStart(0) 1519 } 1520 systemstack(stopTheWorldWithSema) 1521 // The gcphase is _GCmark, it will transition to _GCmarktermination 1522 // below. The important thing is that the wb remains active until 1523 // all marking is complete. This includes writes made by the GC. 1524 1525 if debugCachedWork { 1526 // For debugging, double check that no work was added after we 1527 // went around above and disable write barrier buffering. 1528 for _, p := range allp { 1529 gcw := &p.gcw 1530 if !gcw.empty() { 1531 printlock() 1532 print("runtime: P ", p.id, " flushedWork ", gcw.flushedWork) 1533 if gcw.wbuf1 == nil { 1534 print(" wbuf1=<nil>") 1535 } else { 1536 print(" wbuf1.n=", gcw.wbuf1.nobj) 1537 } 1538 if gcw.wbuf2 == nil { 1539 print(" wbuf2=<nil>") 1540 } else { 1541 print(" wbuf2.n=", gcw.wbuf2.nobj) 1542 } 1543 print("\n") 1544 if gcw.pauseGen == gcw.putGen { 1545 println("runtime: checkPut already failed at this generation") 1546 } 1547 throw("throwOnGCWork") 1548 } 1549 } 1550 } else { 1551 // For unknown reasons (see issue #27993), there is 1552 // sometimes work left over when we enter mark 1553 // termination. Detect this and resume concurrent 1554 // mark. This is obviously unfortunate. 1555 // 1556 // Switch to the system stack to call wbBufFlush1, 1557 // though in this case it doesn't matter because we're 1558 // non-preemptible anyway. 1559 restart := false 1560 systemstack(func() { 1561 for _, p := range allp { 1562 wbBufFlush1(p) 1563 if !p.gcw.empty() { 1564 restart = true 1565 break 1566 } 1567 } 1568 }) 1569 if restart { 1570 getg().m.preemptoff = "" 1571 systemstack(func() { 1572 now := startTheWorldWithSema(true) 1573 work.pauseNS += now - work.pauseStart 1574 }) 1575 goto top 1576 } 1577 } 1578 1579 // Disable assists and background workers. We must do 1580 // this before waking blocked assists. 1581 atomic.Store(&gcBlackenEnabled, 0) 1582 1583 // Wake all blocked assists. These will run when we 1584 // start the world again. 1585 gcWakeAllAssists() 1586 1587 // Likewise, release the transition lock. Blocked 1588 // workers and assists will run when we start the 1589 // world again. 1590 semrelease(&work.markDoneSema) 1591 1592 // In STW mode, re-enable user goroutines. These will be 1593 // queued to run after we start the world. 1594 schedEnableUser(true) 1595 1596 // endCycle depends on all gcWork cache stats being flushed. 1597 // The termination algorithm above ensured that up to 1598 // allocations since the ragged barrier. 1599 nextTriggerRatio := gcController.endCycle() 1600 1601 // Perform mark termination. This will restart the world. 1602 gcMarkTermination(nextTriggerRatio) 1603} 1604 1605func gcMarkTermination(nextTriggerRatio float64) { 1606 // World is stopped. 1607 // Start marktermination which includes enabling the write barrier. 1608 atomic.Store(&gcBlackenEnabled, 0) 1609 setGCPhase(_GCmarktermination) 1610 1611 work.heap1 = memstats.heap_live 1612 startTime := nanotime() 1613 1614 mp := acquirem() 1615 mp.preemptoff = "gcing" 1616 _g_ := getg() 1617 _g_.m.traceback = 2 1618 gp := _g_.m.curg 1619 casgstatus(gp, _Grunning, _Gwaiting) 1620 gp.waitreason = waitReasonGarbageCollection 1621 1622 // Run gc on the g0 stack. We do this so that the g stack 1623 // we're currently running on will no longer change. Cuts 1624 // the root set down a bit (g0 stacks are not scanned, and 1625 // we don't need to scan gc's internal state). We also 1626 // need to switch to g0 so we can shrink the stack. 1627 systemstack(func() { 1628 gcMark(startTime) 1629 // Must return immediately. 1630 // The outer function's stack may have moved 1631 // during gcMark (it shrinks stacks, including the 1632 // outer function's stack), so we must not refer 1633 // to any of its variables. Return back to the 1634 // non-system stack to pick up the new addresses 1635 // before continuing. 1636 }) 1637 1638 systemstack(func() { 1639 work.heap2 = work.bytesMarked 1640 if debug.gccheckmark > 0 { 1641 // Run a full non-parallel, stop-the-world 1642 // mark using checkmark bits, to check that we 1643 // didn't forget to mark anything during the 1644 // concurrent mark process. 1645 gcResetMarkState() 1646 initCheckmarks() 1647 gcw := &getg().m.p.ptr().gcw 1648 gcDrain(gcw, 0) 1649 wbBufFlush1(getg().m.p.ptr()) 1650 gcw.dispose() 1651 clearCheckmarks() 1652 } 1653 1654 // marking is complete so we can turn the write barrier off 1655 setGCPhase(_GCoff) 1656 gcSweep(work.mode) 1657 }) 1658 1659 _g_.m.traceback = 0 1660 casgstatus(gp, _Gwaiting, _Grunning) 1661 1662 if trace.enabled { 1663 traceGCDone() 1664 } 1665 1666 // all done 1667 mp.preemptoff = "" 1668 1669 if gcphase != _GCoff { 1670 throw("gc done but gcphase != _GCoff") 1671 } 1672 1673 // Record next_gc and heap_inuse for scavenger. 1674 memstats.last_next_gc = memstats.next_gc 1675 memstats.last_heap_inuse = memstats.heap_inuse 1676 1677 // Update GC trigger and pacing for the next cycle. 1678 gcSetTriggerRatio(nextTriggerRatio) 1679 1680 // Pacing changed, so the scavenger should be awoken. 1681 wakeScavenger() 1682 1683 // Update timing memstats 1684 now := nanotime() 1685 sec, nsec, _ := time_now() 1686 unixNow := sec*1e9 + int64(nsec) 1687 work.pauseNS += now - work.pauseStart 1688 work.tEnd = now 1689 atomic.Store64(&memstats.last_gc_unix, uint64(unixNow)) // must be Unix time to make sense to user 1690 atomic.Store64(&memstats.last_gc_nanotime, uint64(now)) // monotonic time for us 1691 memstats.pause_ns[memstats.numgc%uint32(len(memstats.pause_ns))] = uint64(work.pauseNS) 1692 memstats.pause_end[memstats.numgc%uint32(len(memstats.pause_end))] = uint64(unixNow) 1693 memstats.pause_total_ns += uint64(work.pauseNS) 1694 1695 // Update work.totaltime. 1696 sweepTermCpu := int64(work.stwprocs) * (work.tMark - work.tSweepTerm) 1697 // We report idle marking time below, but omit it from the 1698 // overall utilization here since it's "free". 1699 markCpu := gcController.assistTime + gcController.dedicatedMarkTime + gcController.fractionalMarkTime 1700 markTermCpu := int64(work.stwprocs) * (work.tEnd - work.tMarkTerm) 1701 cycleCpu := sweepTermCpu + markCpu + markTermCpu 1702 work.totaltime += cycleCpu 1703 1704 // Compute overall GC CPU utilization. 1705 totalCpu := sched.totaltime + (now-sched.procresizetime)*int64(gomaxprocs) 1706 memstats.gc_cpu_fraction = float64(work.totaltime) / float64(totalCpu) 1707 1708 // Reset sweep state. 1709 sweep.nbgsweep = 0 1710 sweep.npausesweep = 0 1711 1712 if work.userForced { 1713 memstats.numforcedgc++ 1714 } 1715 1716 // Bump GC cycle count and wake goroutines waiting on sweep. 1717 lock(&work.sweepWaiters.lock) 1718 memstats.numgc++ 1719 injectglist(&work.sweepWaiters.list) 1720 unlock(&work.sweepWaiters.lock) 1721 1722 // Finish the current heap profiling cycle and start a new 1723 // heap profiling cycle. We do this before starting the world 1724 // so events don't leak into the wrong cycle. 1725 mProf_NextCycle() 1726 1727 systemstack(func() { startTheWorldWithSema(true) }) 1728 1729 // Flush the heap profile so we can start a new cycle next GC. 1730 // This is relatively expensive, so we don't do it with the 1731 // world stopped. 1732 mProf_Flush() 1733 1734 // Prepare workbufs for freeing by the sweeper. We do this 1735 // asynchronously because it can take non-trivial time. 1736 prepareFreeWorkbufs() 1737 1738 // Free stack spans. This must be done between GC cycles. 1739 systemstack(freeStackSpans) 1740 1741 // Ensure all mcaches are flushed. Each P will flush its own 1742 // mcache before allocating, but idle Ps may not. Since this 1743 // is necessary to sweep all spans, we need to ensure all 1744 // mcaches are flushed before we start the next GC cycle. 1745 systemstack(func() { 1746 forEachP(func(_p_ *p) { 1747 _p_.mcache.prepareForSweep() 1748 }) 1749 }) 1750 1751 // Print gctrace before dropping worldsema. As soon as we drop 1752 // worldsema another cycle could start and smash the stats 1753 // we're trying to print. 1754 if debug.gctrace > 0 { 1755 util := int(memstats.gc_cpu_fraction * 100) 1756 1757 var sbuf [24]byte 1758 printlock() 1759 print("gc ", memstats.numgc, 1760 " @", string(itoaDiv(sbuf[:], uint64(work.tSweepTerm-runtimeInitTime)/1e6, 3)), "s ", 1761 util, "%: ") 1762 prev := work.tSweepTerm 1763 for i, ns := range []int64{work.tMark, work.tMarkTerm, work.tEnd} { 1764 if i != 0 { 1765 print("+") 1766 } 1767 print(string(fmtNSAsMS(sbuf[:], uint64(ns-prev)))) 1768 prev = ns 1769 } 1770 print(" ms clock, ") 1771 for i, ns := range []int64{sweepTermCpu, gcController.assistTime, gcController.dedicatedMarkTime + gcController.fractionalMarkTime, gcController.idleMarkTime, markTermCpu} { 1772 if i == 2 || i == 3 { 1773 // Separate mark time components with /. 1774 print("/") 1775 } else if i != 0 { 1776 print("+") 1777 } 1778 print(string(fmtNSAsMS(sbuf[:], uint64(ns)))) 1779 } 1780 print(" ms cpu, ", 1781 work.heap0>>20, "->", work.heap1>>20, "->", work.heap2>>20, " MB, ", 1782 work.heapGoal>>20, " MB goal, ", 1783 work.maxprocs, " P") 1784 if work.userForced { 1785 print(" (forced)") 1786 } 1787 print("\n") 1788 printunlock() 1789 } 1790 1791 semrelease(&worldsema) 1792 // Careful: another GC cycle may start now. 1793 1794 releasem(mp) 1795 mp = nil 1796 1797 // now that gc is done, kick off finalizer thread if needed 1798 if !concurrentSweep { 1799 // give the queued finalizers, if any, a chance to run 1800 Gosched() 1801 } 1802} 1803 1804// gcBgMarkStartWorkers prepares background mark worker goroutines. 1805// These goroutines will not run until the mark phase, but they must 1806// be started while the work is not stopped and from a regular G 1807// stack. The caller must hold worldsema. 1808func gcBgMarkStartWorkers() { 1809 // Background marking is performed by per-P G's. Ensure that 1810 // each P has a background GC G. 1811 for _, p := range allp { 1812 if p.gcBgMarkWorker == 0 { 1813 go gcBgMarkWorker(p) 1814 notetsleepg(&work.bgMarkReady, -1) 1815 noteclear(&work.bgMarkReady) 1816 } 1817 } 1818} 1819 1820// gcBgMarkPrepare sets up state for background marking. 1821// Mutator assists must not yet be enabled. 1822func gcBgMarkPrepare() { 1823 // Background marking will stop when the work queues are empty 1824 // and there are no more workers (note that, since this is 1825 // concurrent, this may be a transient state, but mark 1826 // termination will clean it up). Between background workers 1827 // and assists, we don't really know how many workers there 1828 // will be, so we pretend to have an arbitrarily large number 1829 // of workers, almost all of which are "waiting". While a 1830 // worker is working it decrements nwait. If nproc == nwait, 1831 // there are no workers. 1832 work.nproc = ^uint32(0) 1833 work.nwait = ^uint32(0) 1834} 1835 1836func gcBgMarkWorker(_p_ *p) { 1837 gp := getg() 1838 1839 type parkInfo struct { 1840 m muintptr // Release this m on park. 1841 attach puintptr // If non-nil, attach to this p on park. 1842 } 1843 // We pass park to a gopark unlock function, so it can't be on 1844 // the stack (see gopark). Prevent deadlock from recursively 1845 // starting GC by disabling preemption. 1846 gp.m.preemptoff = "GC worker init" 1847 park := new(parkInfo) 1848 gp.m.preemptoff = "" 1849 1850 park.m.set(acquirem()) 1851 park.attach.set(_p_) 1852 // Inform gcBgMarkStartWorkers that this worker is ready. 1853 // After this point, the background mark worker is scheduled 1854 // cooperatively by gcController.findRunnable. Hence, it must 1855 // never be preempted, as this would put it into _Grunnable 1856 // and put it on a run queue. Instead, when the preempt flag 1857 // is set, this puts itself into _Gwaiting to be woken up by 1858 // gcController.findRunnable at the appropriate time. 1859 notewakeup(&work.bgMarkReady) 1860 1861 for { 1862 // Go to sleep until woken by gcController.findRunnable. 1863 // We can't releasem yet since even the call to gopark 1864 // may be preempted. 1865 gopark(func(g *g, parkp unsafe.Pointer) bool { 1866 park := (*parkInfo)(parkp) 1867 1868 // The worker G is no longer running, so it's 1869 // now safe to allow preemption. 1870 releasem(park.m.ptr()) 1871 1872 // If the worker isn't attached to its P, 1873 // attach now. During initialization and after 1874 // a phase change, the worker may have been 1875 // running on a different P. As soon as we 1876 // attach, the owner P may schedule the 1877 // worker, so this must be done after the G is 1878 // stopped. 1879 if park.attach != 0 { 1880 p := park.attach.ptr() 1881 park.attach.set(nil) 1882 // cas the worker because we may be 1883 // racing with a new worker starting 1884 // on this P. 1885 if !p.gcBgMarkWorker.cas(0, guintptr(unsafe.Pointer(g))) { 1886 // The P got a new worker. 1887 // Exit this worker. 1888 return false 1889 } 1890 } 1891 return true 1892 }, unsafe.Pointer(park), waitReasonGCWorkerIdle, traceEvGoBlock, 0) 1893 1894 // Loop until the P dies and disassociates this 1895 // worker (the P may later be reused, in which case 1896 // it will get a new worker) or we failed to associate. 1897 if _p_.gcBgMarkWorker.ptr() != gp { 1898 break 1899 } 1900 1901 // Disable preemption so we can use the gcw. If the 1902 // scheduler wants to preempt us, we'll stop draining, 1903 // dispose the gcw, and then preempt. 1904 park.m.set(acquirem()) 1905 1906 if gcBlackenEnabled == 0 { 1907 throw("gcBgMarkWorker: blackening not enabled") 1908 } 1909 1910 startTime := nanotime() 1911 _p_.gcMarkWorkerStartTime = startTime 1912 1913 decnwait := atomic.Xadd(&work.nwait, -1) 1914 if decnwait == work.nproc { 1915 println("runtime: work.nwait=", decnwait, "work.nproc=", work.nproc) 1916 throw("work.nwait was > work.nproc") 1917 } 1918 1919 systemstack(func() { 1920 // Mark our goroutine preemptible so its stack 1921 // can be scanned. This lets two mark workers 1922 // scan each other (otherwise, they would 1923 // deadlock). We must not modify anything on 1924 // the G stack. However, stack shrinking is 1925 // disabled for mark workers, so it is safe to 1926 // read from the G stack. 1927 casgstatus(gp, _Grunning, _Gwaiting) 1928 switch _p_.gcMarkWorkerMode { 1929 default: 1930 throw("gcBgMarkWorker: unexpected gcMarkWorkerMode") 1931 case gcMarkWorkerDedicatedMode: 1932 gcDrain(&_p_.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit) 1933 if gp.preempt { 1934 // We were preempted. This is 1935 // a useful signal to kick 1936 // everything out of the run 1937 // queue so it can run 1938 // somewhere else. 1939 lock(&sched.lock) 1940 for { 1941 gp, _ := runqget(_p_) 1942 if gp == nil { 1943 break 1944 } 1945 globrunqput(gp) 1946 } 1947 unlock(&sched.lock) 1948 } 1949 // Go back to draining, this time 1950 // without preemption. 1951 gcDrain(&_p_.gcw, gcDrainFlushBgCredit) 1952 case gcMarkWorkerFractionalMode: 1953 gcDrain(&_p_.gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit) 1954 case gcMarkWorkerIdleMode: 1955 gcDrain(&_p_.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit) 1956 } 1957 casgstatus(gp, _Gwaiting, _Grunning) 1958 }) 1959 1960 // Account for time. 1961 duration := nanotime() - startTime 1962 switch _p_.gcMarkWorkerMode { 1963 case gcMarkWorkerDedicatedMode: 1964 atomic.Xaddint64(&gcController.dedicatedMarkTime, duration) 1965 atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 1) 1966 case gcMarkWorkerFractionalMode: 1967 atomic.Xaddint64(&gcController.fractionalMarkTime, duration) 1968 atomic.Xaddint64(&_p_.gcFractionalMarkTime, duration) 1969 case gcMarkWorkerIdleMode: 1970 atomic.Xaddint64(&gcController.idleMarkTime, duration) 1971 } 1972 1973 // Was this the last worker and did we run out 1974 // of work? 1975 incnwait := atomic.Xadd(&work.nwait, +1) 1976 if incnwait > work.nproc { 1977 println("runtime: p.gcMarkWorkerMode=", _p_.gcMarkWorkerMode, 1978 "work.nwait=", incnwait, "work.nproc=", work.nproc) 1979 throw("work.nwait > work.nproc") 1980 } 1981 1982 // If this worker reached a background mark completion 1983 // point, signal the main GC goroutine. 1984 if incnwait == work.nproc && !gcMarkWorkAvailable(nil) { 1985 // Make this G preemptible and disassociate it 1986 // as the worker for this P so 1987 // findRunnableGCWorker doesn't try to 1988 // schedule it. 1989 _p_.gcBgMarkWorker.set(nil) 1990 releasem(park.m.ptr()) 1991 1992 gcMarkDone() 1993 1994 // Disable preemption and prepare to reattach 1995 // to the P. 1996 // 1997 // We may be running on a different P at this 1998 // point, so we can't reattach until this G is 1999 // parked. 2000 park.m.set(acquirem()) 2001 park.attach.set(_p_) 2002 } 2003 } 2004} 2005 2006// gcMarkWorkAvailable reports whether executing a mark worker 2007// on p is potentially useful. p may be nil, in which case it only 2008// checks the global sources of work. 2009func gcMarkWorkAvailable(p *p) bool { 2010 if p != nil && !p.gcw.empty() { 2011 return true 2012 } 2013 if !work.full.empty() { 2014 return true // global work available 2015 } 2016 if work.markrootNext < work.markrootJobs { 2017 return true // root scan work available 2018 } 2019 return false 2020} 2021 2022// gcMark runs the mark (or, for concurrent GC, mark termination) 2023// All gcWork caches must be empty. 2024// STW is in effect at this point. 2025func gcMark(start_time int64) { 2026 if debug.allocfreetrace > 0 { 2027 tracegc() 2028 } 2029 2030 if gcphase != _GCmarktermination { 2031 throw("in gcMark expecting to see gcphase as _GCmarktermination") 2032 } 2033 work.tstart = start_time 2034 2035 // Check that there's no marking work remaining. 2036 if work.full != 0 || work.markrootNext < work.markrootJobs { 2037 print("runtime: full=", hex(work.full), " next=", work.markrootNext, " jobs=", work.markrootJobs, " nDataRoots=", work.nDataRoots, " nBSSRoots=", work.nBSSRoots, " nSpanRoots=", work.nSpanRoots, " nStackRoots=", work.nStackRoots, "\n") 2038 panic("non-empty mark queue after concurrent mark") 2039 } 2040 2041 if debug.gccheckmark > 0 { 2042 // This is expensive when there's a large number of 2043 // Gs, so only do it if checkmark is also enabled. 2044 gcMarkRootCheck() 2045 } 2046 if work.full != 0 { 2047 throw("work.full != 0") 2048 } 2049 2050 // Clear out buffers and double-check that all gcWork caches 2051 // are empty. This should be ensured by gcMarkDone before we 2052 // enter mark termination. 2053 // 2054 // TODO: We could clear out buffers just before mark if this 2055 // has a non-negligible impact on STW time. 2056 for _, p := range allp { 2057 // The write barrier may have buffered pointers since 2058 // the gcMarkDone barrier. However, since the barrier 2059 // ensured all reachable objects were marked, all of 2060 // these must be pointers to black objects. Hence we 2061 // can just discard the write barrier buffer. 2062 if debug.gccheckmark > 0 || throwOnGCWork { 2063 // For debugging, flush the buffer and make 2064 // sure it really was all marked. 2065 wbBufFlush1(p) 2066 } else { 2067 p.wbBuf.reset() 2068 } 2069 2070 gcw := &p.gcw 2071 if !gcw.empty() { 2072 printlock() 2073 print("runtime: P ", p.id, " flushedWork ", gcw.flushedWork) 2074 if gcw.wbuf1 == nil { 2075 print(" wbuf1=<nil>") 2076 } else { 2077 print(" wbuf1.n=", gcw.wbuf1.nobj) 2078 } 2079 if gcw.wbuf2 == nil { 2080 print(" wbuf2=<nil>") 2081 } else { 2082 print(" wbuf2.n=", gcw.wbuf2.nobj) 2083 } 2084 print("\n") 2085 throw("P has cached GC work at end of mark termination") 2086 } 2087 // There may still be cached empty buffers, which we 2088 // need to flush since we're going to free them. Also, 2089 // there may be non-zero stats because we allocated 2090 // black after the gcMarkDone barrier. 2091 gcw.dispose() 2092 } 2093 2094 throwOnGCWork = false 2095 2096 cachestats() 2097 2098 // Update the marked heap stat. 2099 memstats.heap_marked = work.bytesMarked 2100 2101 // Update other GC heap size stats. This must happen after 2102 // cachestats (which flushes local statistics to these) and 2103 // flushallmcaches (which modifies heap_live). 2104 memstats.heap_live = work.bytesMarked 2105 memstats.heap_scan = uint64(gcController.scanWork) 2106 2107 if trace.enabled { 2108 traceHeapAlloc() 2109 } 2110} 2111 2112// gcSweep must be called on the system stack because it acquires the heap 2113// lock. See mheap for details. 2114//go:systemstack 2115func gcSweep(mode gcMode) { 2116 if gcphase != _GCoff { 2117 throw("gcSweep being done but phase is not GCoff") 2118 } 2119 2120 lock(&mheap_.lock) 2121 mheap_.sweepgen += 2 2122 mheap_.sweepdone = 0 2123 if mheap_.sweepSpans[mheap_.sweepgen/2%2].index != 0 { 2124 // We should have drained this list during the last 2125 // sweep phase. We certainly need to start this phase 2126 // with an empty swept list. 2127 throw("non-empty swept list") 2128 } 2129 mheap_.pagesSwept = 0 2130 mheap_.sweepArenas = mheap_.allArenas 2131 mheap_.reclaimIndex = 0 2132 mheap_.reclaimCredit = 0 2133 unlock(&mheap_.lock) 2134 2135 if !_ConcurrentSweep || mode == gcForceBlockMode { 2136 // Special case synchronous sweep. 2137 // Record that no proportional sweeping has to happen. 2138 lock(&mheap_.lock) 2139 mheap_.sweepPagesPerByte = 0 2140 unlock(&mheap_.lock) 2141 // Sweep all spans eagerly. 2142 for sweepone() != ^uintptr(0) { 2143 sweep.npausesweep++ 2144 } 2145 // Free workbufs eagerly. 2146 prepareFreeWorkbufs() 2147 for freeSomeWbufs(false) { 2148 } 2149 // All "free" events for this mark/sweep cycle have 2150 // now happened, so we can make this profile cycle 2151 // available immediately. 2152 mProf_NextCycle() 2153 mProf_Flush() 2154 return 2155 } 2156 2157 // Background sweep. 2158 lock(&sweep.lock) 2159 if sweep.parked { 2160 sweep.parked = false 2161 ready(sweep.g, 0, true) 2162 } 2163 unlock(&sweep.lock) 2164} 2165 2166// gcResetMarkState resets global state prior to marking (concurrent 2167// or STW) and resets the stack scan state of all Gs. 2168// 2169// This is safe to do without the world stopped because any Gs created 2170// during or after this will start out in the reset state. 2171// 2172// gcResetMarkState must be called on the system stack because it acquires 2173// the heap lock. See mheap for details. 2174// 2175//go:systemstack 2176func gcResetMarkState() { 2177 // This may be called during a concurrent phase, so make sure 2178 // allgs doesn't change. 2179 lock(&allglock) 2180 for _, gp := range allgs { 2181 gp.gcscandone = false // set to true in gcphasework 2182 gp.gcAssistBytes = 0 2183 } 2184 unlock(&allglock) 2185 2186 // Clear page marks. This is just 1MB per 64GB of heap, so the 2187 // time here is pretty trivial. 2188 lock(&mheap_.lock) 2189 arenas := mheap_.allArenas 2190 unlock(&mheap_.lock) 2191 for _, ai := range arenas { 2192 ha := mheap_.arenas[ai.l1()][ai.l2()] 2193 for i := range ha.pageMarks { 2194 ha.pageMarks[i] = 0 2195 } 2196 } 2197 2198 work.bytesMarked = 0 2199 work.initialHeapLive = atomic.Load64(&memstats.heap_live) 2200} 2201 2202// Hooks for other packages 2203 2204var poolcleanup func() 2205 2206//go:linkname sync_runtime_registerPoolCleanup sync.runtime_registerPoolCleanup 2207func sync_runtime_registerPoolCleanup(f func()) { 2208 poolcleanup = f 2209} 2210 2211func clearpools() { 2212 // clear sync.Pools 2213 if poolcleanup != nil { 2214 poolcleanup() 2215 } 2216 2217 // Clear central sudog cache. 2218 // Leave per-P caches alone, they have strictly bounded size. 2219 // Disconnect cached list before dropping it on the floor, 2220 // so that a dangling ref to one entry does not pin all of them. 2221 lock(&sched.sudoglock) 2222 var sg, sgnext *sudog 2223 for sg = sched.sudogcache; sg != nil; sg = sgnext { 2224 sgnext = sg.next 2225 sg.next = nil 2226 } 2227 sched.sudogcache = nil 2228 unlock(&sched.sudoglock) 2229 2230 // Clear central defer pools. 2231 // Leave per-P pools alone, they have strictly bounded size. 2232 lock(&sched.deferlock) 2233 for i := range sched.deferpool { 2234 // disconnect cached list before dropping it on the floor, 2235 // so that a dangling ref to one entry does not pin all of them. 2236 var d, dlink *_defer 2237 for d = sched.deferpool[i]; d != nil; d = dlink { 2238 dlink = d.link 2239 d.link = nil 2240 } 2241 sched.deferpool[i] = nil 2242 } 2243 unlock(&sched.deferlock) 2244} 2245 2246// Timing 2247 2248// itoaDiv formats val/(10**dec) into buf. 2249func itoaDiv(buf []byte, val uint64, dec int) []byte { 2250 i := len(buf) - 1 2251 idec := i - dec 2252 for val >= 10 || i >= idec { 2253 buf[i] = byte(val%10 + '0') 2254 i-- 2255 if i == idec { 2256 buf[i] = '.' 2257 i-- 2258 } 2259 val /= 10 2260 } 2261 buf[i] = byte(val + '0') 2262 return buf[i:] 2263} 2264 2265// fmtNSAsMS nicely formats ns nanoseconds as milliseconds. 2266func fmtNSAsMS(buf []byte, ns uint64) []byte { 2267 if ns >= 10e6 { 2268 // Format as whole milliseconds. 2269 return itoaDiv(buf, ns/1e6, 0) 2270 } 2271 // Format two digits of precision, with at most three decimal places. 2272 x := ns / 1e3 2273 if x == 0 { 2274 buf[0] = '0' 2275 return buf[:1] 2276 } 2277 dec := 3 2278 for x >= 100 { 2279 x /= 10 2280 dec-- 2281 } 2282 return itoaDiv(buf, x, dec) 2283} 2284