1A zygote process is one that listens for spawn requests from a master process 2and forks itself in response. Generally they are used because forking a process 3after some expensive setup has been performed can save time and share extra 4memory pages. 5 6More specifically, on Linux, it allows to: 7 * Amortize the runtime and memory cost of the dynamic loader's relocations, 8 which is respectively ~6 MB and 60 ms/GHz per process. 9 See [Appendix A](#appendix-a-runtime-impact-of-relocations) and 10 [Appendix B](#appendix-b-memory-impact-of-relocations). 11 * Amortize the runtime and memory cost for initializing common 12 libraries, such as ICU, NSS, the V8 snapshot and anything else in 13 `ContentMainRunnerImpl::Initialize()`. With the above, this saves 14 up to ~8 MB per process. See [Appendix C](#appendix-c-overall-memory-impact). 15 16Security-wise, the Zygote is responsible for setting up and bookkeeping the 17[namespace sandbox](sandboxing.md). 18 19Furthermore it is the only reasonable way to keep a reference to a binary 20and a set of shared libraries that can be exec'ed. In the model used on Windows 21and Mac, renderers are exec'ed as needed from the chrome binary. However, if the 22chrome binary, or any of its shared libraries are updated while Chrome is 23running, we'll end up exec'ing the wrong version. A version _x_ browser might be 24talking to a version _y_ renderer. Our IPC system does not support this (and 25does not want to!). 26 27So we would like to keep a reference to a binary and its shared libraries and 28exec from these. However, unless we are going to write our own `ld.so`, there's 29no way to do this. 30 31Instead, we exec the prototypical renderer at the beginning of the browser 32execution. When we need more renderers, we signal this prototypical process (the 33zygote) to fork itself. The zygote is always the correct version and, by 34exec'ing one, we make sure the renderers have a different address space 35randomisation than the browser. 36 37The zygote process is triggered by the `--type=zygote` command line flag, which 38causes `ZygoteMain` (in `chrome/browser/zygote_main_linux.cc`) to be run. The 39zygote is launched from `content/browser/zygote_host/zygote_host_impl_linux.cc`. 40 41Signaling the zygote for a new renderer happens in 42`chrome/browser/child_process_launcher.cc`. 43 44You can use the `--zygote-cmd-prefix` flag to debug the zygote process. If you 45use `--renderer-cmd-prefix` then the zygote will be bypassed and renderers will 46be exec'ed afresh every time. 47 48## Appendix A: Runtime impact of relocations 49Measured on a Z620: 50 51 $ LD_DEBUG=statistics /opt/google/chrome-beta/chrome --help 52 runtime linker statistics: 53 total startup time in dynamic loader: 73899158 clock cycles 54 time needed for relocation: 56836478 clock cycles (76.9%) 55 number of relocations: 4271 56 number of relocations from cache: 11347 57 number of relative relocations: 502740 58 time needed to load objects: 15789844 clock cycles (21.3%) 59 6056836478 clock cycles -> ~56 ms/GHz 61 62## Appendix B: Memory impact of relocations 63 64 $ readelf -WS /opt/google/chrome-beta/chrome 65 [Nr] Name Type Address Off Size ES Flg Lk Inf Al 66 ... 67 [25] .data.rel.ro PROGBITS 0000000006a8b590 6a8a590 5b5500 00 WA 0 0 16 68 ... 69 Note: 0x5b5500 -> 5.98 MB 70 71Actual impact in terms of memory pages that get shared due to CoW: 72 73 $ cat /proc/.../smaps 74 7fbdd1c81000-7fbdd2233000 r--p 06a5d000 fc:00 665771 /opt/google/chrome-unstable/chrome 75 ... 76 Shared_Dirty: 5796 kB 77 78## Appendix C: Overall memory impact 79 $ cat /proc/$PID_OF_ZYGOTE/smaps | grep Shared_Dirty | awk '{TOTAL += $2} END {print TOTAL}' 80 8092 # KB for dirty pages shared with other processes (mostly forked child processes). 81