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runtime/pprof: document block and mutex profiles

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Amazingly, we seem to have nearly no in-tree documentation on the
semantics of block and mutex profiles. Add brief summaries, including
the new behavior from CL 506415 and CL 544195.

For #14689.
For #44920.
For #57071.
For #61015.

Change-Id: I1a6edce7c434fcb43f17c83eb362b1f9d1a32df1
Reviewed-on: https://go-review.googlesource.com/c/go/+/547057
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Rhys Hiltner <rhys@justin.tv>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
This commit is contained in:
Michael Pratt 2023-12-04 15:13:13 -05:00 committed by Gopher Robot
parent 71fc9d4da5
commit e9eb2beeb2
1 changed files with 25 additions and 0 deletions
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25
src/runtime/pprof/pprof.go
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@ -126,6 +126,31 @@ import (
// pprof display to -alloc_space, the total number of bytes allocated since
// the program began (including garbage-collected bytes).
//
// The block profile tracks time spent blocked on synchronization primitives,
// such as [sync.Mutex], [sync.RWMutex], [sync.WaitGroup], [sync.Cond], and
// channel send/receive/select. Stack traces correspond to the location that
// blocked (for example, [sync.Mutex.Lock]). Sample values correspond to
// cumulative time spent blocked at that stack trace, subject to time-based
// sampling specified by [runtime.SetBlockProfileRate].
//
// The mutex profile tracks contention on mutexes, such as [sync.Mutex],
// [sync.RWMutex], and runtime-internal locks. Stack traces correspond to the
// end of the critical section causing contention. For example, a lock held for
// a long time while other goroutines are waiting to acquire the lock will
// report contention when the lock is finally unlocked (that is, at
// [sync.Mutex.Unlock]). Sample values correspond to the approximate cumulative
// time other goroutines spent blocked waiting for the lock, subject to
// event-based sampling specified by [runtime.SetMutexProfileFraction]. For
// example, if a caller holds a lock for 1s while 5 other goroutines are
// waiting for the entire second to acquire the lock, its unlock call stack
// will report 5s of contention.
//
// In the mutex profile, runtime-internal locks are always reported at the
// location "runtime._LostContendedRuntimeLock". More detailed stack traces for
// runtime-internal locks can be obtained by setting
// `GODEBUG=runtimecontentionstacks=1` (see package [runtime] docs for
// caveats).
//
// The CPU profile is not available as a Profile. It has a special API,
// the [StartCPUProfile] and [StopCPUProfile] functions, because it streams
// output to a writer during profiling.