1 /*------------------------------------------------------------------------- 2 * 3 * bgwriter.c 4 * 5 * The background writer (bgwriter) is new as of Postgres 8.0. It attempts 6 * to keep regular backends from having to write out dirty shared buffers 7 * (which they would only do when needing to free a shared buffer to read in 8 * another page). In the best scenario all writes from shared buffers will 9 * be issued by the background writer process. However, regular backends are 10 * still empowered to issue writes if the bgwriter fails to maintain enough 11 * clean shared buffers. 12 * 13 * As of Postgres 9.2 the bgwriter no longer handles checkpoints. 14 * 15 * The bgwriter is started by the postmaster as soon as the startup subprocess 16 * finishes, or as soon as recovery begins if we are doing archive recovery. 17 * It remains alive until the postmaster commands it to terminate. 18 * Normal termination is by SIGTERM, which instructs the bgwriter to exit(0). TEST(LlvmLibccosffExhaustiveTest,AllValues)19 * Emergency termination is by SIGQUIT; like any backend, the bgwriter will 20 * simply abort and exit on SIGQUIT. 21 * 22 * If the bgwriter exits unexpectedly, the postmaster treats that the same 23 * as a backend crash: shared memory may be corrupted, so remaining backends 24 * should be killed by SIGQUIT and then a recovery cycle started. 25 * 26 * 27 * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group 28 * 29 * 30 * IDENTIFICATION 31 * src/backend/postmaster/bgwriter.c 32 * 33 *------------------------------------------------------------------------- 34 */ 35 #include "postgres.h" 36 37 #include <signal.h> 38 #include <sys/time.h> 39 #include <unistd.h> 40 41 #include "access/xlog.h" 42 #include "access/xlog_internal.h" 43 #include "libpq/pqsignal.h" 44 #include "miscadmin.h" 45 #include "pgstat.h" 46 #include "postmaster/bgwriter.h" 47 #include "storage/bufmgr.h" 48 #include "storage/buf_internals.h" 49 #include "storage/condition_variable.h" 50 #include "storage/fd.h" 51 #include "storage/ipc.h" 52 #include "storage/lwlock.h" 53 #include "storage/proc.h" 54 #include "storage/shmem.h" 55 #include "storage/smgr.h" 56 #include "storage/spin.h" 57 #include "storage/standby.h" 58 #include "utils/guc.h" 59 #include "utils/memutils.h" 60 #include "utils/resowner.h" 61 #include "utils/timestamp.h" 62 63 64 /* 65 * GUC parameters 66 */ 67 int BgWriterDelay = 200; 68 69 /* 70 * Multiplier to apply to BgWriterDelay when we decide to hibernate. 71 * (Perhaps this needs to be configurable?) 72 */ 73 #define HIBERNATE_FACTOR 50 74 75 /* 76 * Interval in which standby snapshots are logged into the WAL stream, in 77 * milliseconds. 78 */ 79 #define LOG_SNAPSHOT_INTERVAL_MS 15000 80 81 /* 82 * LSN and timestamp at which we last issued a LogStandbySnapshot(), to avoid 83 * doing so too often or repeatedly if there has been no other write activity 84 * in the system. 85 */ 86 static TimestampTz last_snapshot_ts; 87 static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr; 88 89 /* 90 * Flags set by interrupt handlers for later service in the main loop. 91 */ 92 static volatile sig_atomic_t got_SIGHUP = false; 93 static volatile sig_atomic_t shutdown_requested = false; 94 95 /* Signal handlers */ 96 97 static void bg_quickdie(SIGNAL_ARGS); 98 static void BgSigHupHandler(SIGNAL_ARGS); 99 static void ReqShutdownHandler(SIGNAL_ARGS); 100 static void bgwriter_sigusr1_handler(SIGNAL_ARGS); 101 102 103 /* 104 * Main entry point for bgwriter process 105 * 106 * This is invoked from AuxiliaryProcessMain, which has already created the 107 * basic execution environment, but not enabled signals yet. 108 */ 109 void 110 BackgroundWriterMain(void) 111 { 112 sigjmp_buf local_sigjmp_buf; 113 MemoryContext bgwriter_context; 114 bool prev_hibernate; 115 WritebackContext wb_context; 116 117 /* 118 * Properly accept or ignore signals the postmaster might send us. 119 * 120 * bgwriter doesn't participate in ProcSignal signalling, but a SIGUSR1 121 * handler is still needed for latch wakeups. 122 */ 123 pqsignal(SIGHUP, BgSigHupHandler); /* set flag to read config file */ 124 pqsignal(SIGINT, SIG_IGN); 125 pqsignal(SIGTERM, ReqShutdownHandler); /* shutdown */ 126 pqsignal(SIGQUIT, bg_quickdie); /* hard crash time */ 127 pqsignal(SIGALRM, SIG_IGN); 128 pqsignal(SIGPIPE, SIG_IGN); 129 pqsignal(SIGUSR1, bgwriter_sigusr1_handler); 130 pqsignal(SIGUSR2, SIG_IGN); 131 132 /* 133 * Reset some signals that are accepted by postmaster but not here 134 */ 135 pqsignal(SIGCHLD, SIG_DFL); 136 pqsignal(SIGTTIN, SIG_DFL); 137 pqsignal(SIGTTOU, SIG_DFL); 138 pqsignal(SIGCONT, SIG_DFL); 139 pqsignal(SIGWINCH, SIG_DFL); 140 141 /* We allow SIGQUIT (quickdie) at all times */ 142 sigdelset(&BlockSig, SIGQUIT); 143 144 /* 145 * Create a resource owner to keep track of our resources (currently only 146 * buffer pins). 147 */ 148 CurrentResourceOwner = ResourceOwnerCreate(NULL, "Background Writer"); 149 150 /* 151 * We just started, assume there has been either a shutdown or 152 * end-of-recovery snapshot. 153 */ 154 last_snapshot_ts = GetCurrentTimestamp(); 155 156 /* 157 * Create a memory context that we will do all our work in. We do this so 158 * that we can reset the context during error recovery and thereby avoid 159 * possible memory leaks. Formerly this code just ran in 160 * TopMemoryContext, but resetting that would be a really bad idea. 161 */ 162 bgwriter_context = AllocSetContextCreate(TopMemoryContext, 163 "Background Writer", 164 ALLOCSET_DEFAULT_SIZES); 165 MemoryContextSwitchTo(bgwriter_context); 166 167 WritebackContextInit(&wb_context, &bgwriter_flush_after); 168 169 /* 170 * If an exception is encountered, processing resumes here. 171 * 172 * See notes in postgres.c about the design of this coding. 173 */ 174 if (sigsetjmp(local_sigjmp_buf, 1) != 0) 175 { 176 /* Since not using PG_TRY, must reset error stack by hand */ 177 error_context_stack = NULL; 178 179 /* Prevent interrupts while cleaning up */ 180 HOLD_INTERRUPTS(); 181 182 /* Report the error to the server log */ 183 EmitErrorReport(); 184 185 /* 186 * These operations are really just a minimal subset of 187 * AbortTransaction(). We don't have very many resources to worry 188 * about in bgwriter, but we do have LWLocks, buffers, and temp files. 189 */ 190 LWLockReleaseAll(); 191 ConditionVariableCancelSleep(); 192 AbortBufferIO(); 193 UnlockBuffers(); 194 /* buffer pins are released here: */ 195 ResourceOwnerRelease(CurrentResourceOwner, 196 RESOURCE_RELEASE_BEFORE_LOCKS, 197 false, true); 198 /* we needn't bother with the other ResourceOwnerRelease phases */ 199 AtEOXact_Buffers(false); 200 AtEOXact_SMgr(); 201 AtEOXact_Files(false); 202 AtEOXact_HashTables(false); 203 204 /* 205 * Now return to normal top-level context and clear ErrorContext for 206 * next time. 207 */ 208 MemoryContextSwitchTo(bgwriter_context); 209 FlushErrorState(); 210 211 /* Flush any leaked data in the top-level context */ 212 MemoryContextResetAndDeleteChildren(bgwriter_context); 213 214 /* re-initialize to avoid repeated errors causing problems */ 215 WritebackContextInit(&wb_context, &bgwriter_flush_after); 216 217 /* Now we can allow interrupts again */ 218 RESUME_INTERRUPTS(); 219 220 /* 221 * Sleep at least 1 second after any error. A write error is likely 222 * to be repeated, and we don't want to be filling the error logs as 223 * fast as we can. 224 */ 225 pg_usleep(1000000L); 226 227 /* 228 * Close all open files after any error. This is helpful on Windows, 229 * where holding deleted files open causes various strange errors. 230 * It's not clear we need it elsewhere, but shouldn't hurt. 231 */ 232 smgrcloseall(); 233 234 /* Report wait end here, when there is no further possibility of wait */ 235 pgstat_report_wait_end(); 236 } 237 238 /* We can now handle ereport(ERROR) */ 239 PG_exception_stack = &local_sigjmp_buf; 240 241 /* 242 * Unblock signals (they were blocked when the postmaster forked us) 243 */ 244 PG_SETMASK(&UnBlockSig); 245 246 /* 247 * Reset hibernation state after any error. 248 */ 249 prev_hibernate = false; 250 251 /* 252 * Loop forever 253 */ 254 for (;;) 255 { 256 bool can_hibernate; 257 int rc; 258 259 /* Clear any already-pending wakeups */ 260 ResetLatch(MyLatch); 261 262 if (got_SIGHUP) 263 { 264 got_SIGHUP = false; 265 ProcessConfigFile(PGC_SIGHUP); 266 } 267 if (shutdown_requested) 268 { 269 /* 270 * From here on, elog(ERROR) should end with exit(1), not send 271 * control back to the sigsetjmp block above 272 */ 273 ExitOnAnyError = true; 274 /* Normal exit from the bgwriter is here */ 275 proc_exit(0); /* done */ 276 } 277 278 /* 279 * Do one cycle of dirty-buffer writing. 280 */ 281 can_hibernate = BgBufferSync(&wb_context); 282 283 /* 284 * Send off activity statistics to the stats collector 285 */ 286 pgstat_send_bgwriter(); 287 288 if (FirstCallSinceLastCheckpoint()) 289 { 290 /* 291 * After any checkpoint, close all smgr files. This is so we 292 * won't hang onto smgr references to deleted files indefinitely. 293 */ 294 smgrcloseall(); 295 } 296 297 /* 298 * Log a new xl_running_xacts every now and then so replication can 299 * get into a consistent state faster (think of suboverflowed 300 * snapshots) and clean up resources (locks, KnownXids*) more 301 * frequently. The costs of this are relatively low, so doing it 4 302 * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine. 303 * 304 * We assume the interval for writing xl_running_xacts is 305 * significantly bigger than BgWriterDelay, so we don't complicate the 306 * overall timeout handling but just assume we're going to get called 307 * often enough even if hibernation mode is active. It's not that 308 * important that log_snap_interval_ms is met strictly. To make sure 309 * we're not waking the disk up unnecessarily on an idle system we 310 * check whether there has been any WAL inserted since the last time 311 * we've logged a running xacts. 312 * 313 * We do this logging in the bgwriter as it is the only process that 314 * is run regularly and returns to its mainloop all the time. E.g. 315 * Checkpointer, when active, is barely ever in its mainloop and thus 316 * makes it hard to log regularly. 317 */ 318 if (XLogStandbyInfoActive() && !RecoveryInProgress()) 319 { 320 TimestampTz timeout = 0; 321 TimestampTz now = GetCurrentTimestamp(); 322 323 timeout = TimestampTzPlusMilliseconds(last_snapshot_ts, 324 LOG_SNAPSHOT_INTERVAL_MS); 325 326 /* 327 * Only log if enough time has passed and interesting records have 328 * been inserted since the last snapshot. Have to compare with <= 329 * instead of < because GetLastImportantRecPtr() points at the 330 * start of a record, whereas last_snapshot_lsn points just past 331 * the end of the record. 332 */ 333 if (now >= timeout && 334 last_snapshot_lsn <= GetLastImportantRecPtr()) 335 { 336 last_snapshot_lsn = LogStandbySnapshot(); 337 last_snapshot_ts = now; 338 } 339 } 340 341 /* 342 * Sleep until we are signaled or BgWriterDelay has elapsed. 343 * 344 * Note: the feedback control loop in BgBufferSync() expects that we 345 * will call it every BgWriterDelay msec. While it's not critical for 346 * correctness that that be exact, the feedback loop might misbehave 347 * if we stray too far from that. Hence, avoid loading this process 348 * down with latch events that are likely to happen frequently during 349 * normal operation. 350 */ 351 rc = WaitLatch(MyLatch, 352 WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH, 353 BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN); 354 355 /* 356 * If no latch event and BgBufferSync says nothing's happening, extend 357 * the sleep in "hibernation" mode, where we sleep for much longer 358 * than bgwriter_delay says. Fewer wakeups save electricity. When a 359 * backend starts using buffers again, it will wake us up by setting 360 * our latch. Because the extra sleep will persist only as long as no 361 * buffer allocations happen, this should not distort the behavior of 362 * BgBufferSync's control loop too badly; essentially, it will think 363 * that the system-wide idle interval didn't exist. 364 * 365 * There is a race condition here, in that a backend might allocate a 366 * buffer between the time BgBufferSync saw the alloc count as zero 367 * and the time we call StrategyNotifyBgWriter. While it's not 368 * critical that we not hibernate anyway, we try to reduce the odds of 369 * that by only hibernating when BgBufferSync says nothing's happening 370 * for two consecutive cycles. Also, we mitigate any possible 371 * consequences of a missed wakeup by not hibernating forever. 372 */ 373 if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate) 374 { 375 /* Ask for notification at next buffer allocation */ 376 StrategyNotifyBgWriter(MyProc->pgprocno); 377 /* Sleep ... */ 378 rc = WaitLatch(MyLatch, 379 WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH, 380 BgWriterDelay * HIBERNATE_FACTOR, 381 WAIT_EVENT_BGWRITER_HIBERNATE); 382 /* Reset the notification request in case we timed out */ 383 StrategyNotifyBgWriter(-1); 384 } 385 386 /* 387 * Emergency bailout if postmaster has died. This is to avoid the 388 * necessity for manual cleanup of all postmaster children. 389 */ 390 if (rc & WL_POSTMASTER_DEATH) 391 exit(1); 392 393 prev_hibernate = can_hibernate; 394 } 395 } 396 397 398 /* -------------------------------- 399 * signal handler routines 400 * -------------------------------- 401 */ 402 403 /* 404 * bg_quickdie() occurs when signalled SIGQUIT by the postmaster. 405 * 406 * Some backend has bought the farm, 407 * so we need to stop what we're doing and exit. 408 */ 409 static void 410 bg_quickdie(SIGNAL_ARGS) 411 { 412 /* 413 * We DO NOT want to run proc_exit() or atexit() callbacks -- we're here 414 * because shared memory may be corrupted, so we don't want to try to 415 * clean up our transaction. Just nail the windows shut and get out of 416 * town. The callbacks wouldn't be safe to run from a signal handler, 417 * anyway. 418 * 419 * Note we do _exit(2) not _exit(0). This is to force the postmaster into 420 * a system reset cycle if someone sends a manual SIGQUIT to a random 421 * backend. This is necessary precisely because we don't clean up our 422 * shared memory state. (The "dead man switch" mechanism in pmsignal.c 423 * should ensure the postmaster sees this as a crash, too, but no harm in 424 * being doubly sure.) 425 */ 426 _exit(2); 427 } 428 429 /* SIGHUP: set flag to re-read config file at next convenient time */ 430 static void 431 BgSigHupHandler(SIGNAL_ARGS) 432 { 433 int save_errno = errno; 434 435 got_SIGHUP = true; 436 SetLatch(MyLatch); 437 438 errno = save_errno; 439 } 440 441 /* SIGTERM: set flag to shutdown and exit */ 442 static void 443 ReqShutdownHandler(SIGNAL_ARGS) 444 { 445 int save_errno = errno; 446 447 shutdown_requested = true; 448 SetLatch(MyLatch); 449 450 errno = save_errno; 451 } 452 453 /* SIGUSR1: used for latch wakeups */ 454 static void 455 bgwriter_sigusr1_handler(SIGNAL_ARGS) 456 { 457 int save_errno = errno; 458 459 latch_sigusr1_handler(); 460 461 errno = save_errno; 462 } 463