8cfd8c3db8
Using setitimer on Linux to request SIGPROF signal deliveries in proportion to the process's on-CPU time results in under-reporting when the program uses several goroutines in parallel. Linux calculates the process's total CPU spend on a regular basis (often every 4ms); if the process has spent enough CPU time since the last calculation to warrant more than one SIGPROF (usually 10ms for the default sample rate of 100 Hz), the kernel is often able to deliver only one of them. With these common settings, that results in Go CPU profiles being attenuated for programs that use more than 2.5 goroutines in parallel. To avoid in effect overflowing the kernel's process-wide CPU counter, and relying on Linux's typical behavior of having the active thread handle the resulting process-targeted signal, use timer_create to request a timer for each OS thread that the Go runtime manages. Have each timer track the CPU time of a single thread, with the resulting SIGPROF going directly to that thread. To continue tracking CPU time spent on threads that don't interact with the Go runtime (such as those created and used in cgo), keep using setitimer in addition to the new mechanism. When a SIGPROF signal arrives, check whether it's due to setitimer or timer_create and filter as appropriate: If the thread is known to Go (has an M) and has a timer_create timer, ignore SIGPROF signals from setitimer. If the thread is not known to Go (does not have an M), ignore SIGPROF signals that are not from setitimer. Counteract the new bias that per-thread profiling adds against short-lived threads (or those that are only active on occasion for a short time, such as garbage collection workers on mostly-idle systems) by configuring the timers' initial trigger to be from a uniform random distribution between "immediate trigger" and the full requested sample period. Updates #35057 Change-Id: Iab753c4e5101bdc09ef9132eec84a75478e05579 Reviewed-on: https://go-review.googlesource.com/c/go/+/324129 Run-TryBot: Rhys Hiltner <rhys@justin.tv> TryBot-Result: Go Bot <gobot@golang.org> Trust: David Chase <drchase@google.com> Reviewed-by: Michael Pratt <mpratt@google.com> |
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