diff options
Diffstat (limited to 'src/runtime/debug.go')
| -rw-r--r-- | src/runtime/debug.go | 51 |
1 files changed, 43 insertions, 8 deletions
diff --git a/src/runtime/debug.go b/src/runtime/debug.go index c7592d3329..206cf39f02 100644 --- a/src/runtime/debug.go +++ b/src/runtime/debug.go @@ -13,13 +13,23 @@ import ( // simultaneously and returns the previous setting. If n < 1, it does not change // the current setting. // +// # Default +// // If the GOMAXPROCS environment variable is set to a positive whole number, // GOMAXPROCS defaults to that value. // -// Otherwise, the Go runtime selects an appropriate default value based on the -// number of logical CPUs on the machine, the process’s CPU affinity mask, and, -// on Linux, the process’s average CPU throughput limit based on cgroup CPU -// quota, if any. +// Otherwise, the Go runtime selects an appropriate default value from a combination of +// - the number of logical CPUs on the machine, +// - the process’s CPU affinity mask, +// - and, on Linux, the process’s average CPU throughput limit based on cgroup CPU +// quota, if any. +// +// If GODEBUG=containermaxprocs=0 is set and GOMAXPROCS is not set by the +// environment variable, then GOMAXPROCS instead defaults to the value of +// [runtime.NumCPU]. Note that GODEBUG=containermaxprocs=0 is [default] for +// language version 1.24 and below. +// +// # Updates // // The Go runtime periodically updates the default value based on changes to // the total logical CPU count, the CPU affinity mask, or cgroup quota. Setting @@ -27,11 +37,36 @@ import ( // GOMAXPROCS disables automatic updates. The default value and automatic // updates can be restored by calling [SetDefaultGOMAXPROCS]. // -// If GODEBUG=containermaxprocs=0 is set, GOMAXPROCS defaults to the value of -// [runtime.NumCPU]. If GODEBUG=updatemaxprocs=0 is set, the Go runtime does -// not perform automatic GOMAXPROCS updating. +// If GODEBUG=updatemaxprocs=0 is set, the Go runtime does not perform +// automatic GOMAXPROCS updating. Note that GODEBUG=updatemaxprocs=0 is +// [default] for language version 1.24 and below. +// +// # Compatibility +// +// Note that the default GOMAXPROCS behavior may change as the scheduler +// improves, especially the implementation detail below. +// +// # Implementation details +// +// When computing default GOMAXPROCS via cgroups, the Go runtime computes the +// "average CPU throughput limit" as the cgroup CPU quota / period. In cgroup +// v2, these values come from the cpu.max file. In cgroup v1, they come from +// cpu.cfs_quota_us and cpu.cfs_period_us, respectively. In container runtimes +// that allow configuring CPU limits, this value usually corresponds to the +// "CPU limit" option, not "CPU request". +// +// The Go runtime typically selects the default GOMAXPROCS as the minimum of +// the logical CPU count, the CPU affinity mask count, or the cgroup CPU +// throughput limit. However, it will never set GOMAXPROCS less than 2 unless +// the logical CPU count or CPU affinity mask count are below 2. +// +// If the cgroup CPU throughput limit is not a whole number, the Go runtime +// rounds up to the next whole number. +// +// GOMAXPROCS updates are performed up to once per second, or less if the +// application is idle. // -// The default GOMAXPROCS behavior may change as the scheduler improves. +// [default]: https://go.dev/doc/godebug#default func GOMAXPROCS(n int) int { if GOARCH == "wasm" && n > 1 { n = 1 // WebAssembly has no threads yet, so only one CPU is possible. |