ad943066f6
goschedImpl transitions the current goroutine from _Grunning to _Grunnable and places it on the global run queue before calling into schedule. It does _not_ call wakep after adding the global run queue. I believe the intuition behind skipping wakep is that since we are immediately calling the scheduler so we don't need to wake anything to run this work. Unfortunately, this intuition is not correct, as it breaks coordination with spinning Ms [1]. Consider this example scenario: Initial conditions: M0: Running P0, G0 M1: Spinning, holding P1 and looking for work Timeline: M1: Fails to find work; drops P M0: newproc adds G1 to P0 runq M0: does not wakep because there is a spinning M M1: clear mp.spinning, decrement sched.nmspinning (now in "delicate dance") M1: check sched.runqsize -> no global runq work M0: gosched preempts G0; adds G0 to global runq M0: does not wakep because gosched doesn't wakep M0: schedules G1 from P0 runq M1: check P0 runq -> no work M1: no work -> park G0 is stranded on the global runq with no M/P looking to run it. This is a loss of work conservation. As a result, G0 will have unbounded* scheduling delay, only getting scheduled when G1 yields. Even once G1 yields, we still won't start another P, so both G0 and G1 will switch back and forth sharing one P when they should start another. *The caveat to this is that today sysmon will preempt G1 after 10ms, effectively capping the scheduling delay to 10ms, but not solving the P underutilization problem. Sysmon's behavior here is theoretically unnecessary, as our work conservation guarantee should allow sysmon to avoid preemption if there are any idle Ps. Issue #60693 tracks changing this behavior and the challenges involved. [1] It would be OK if we unconditionally entered the scheduler as a spinning M ourselves, as that would require schedule to call wakep when it finds work in case there is more work. Fixes #55160. Change-Id: I2f44001239564b56ea30212553ab557051d22588 Reviewed-on: https://go-review.googlesource.com/c/go/+/501976 Reviewed-by: Michael Knyszek <mknyszek@google.com> TryBot-Result: Gopher Robot <gobot@golang.org> Run-TryBot: Michael Pratt <mpratt@google.com> Auto-Submit: Michael Pratt <mpratt@google.com> |
||
|---|---|---|
| .github | ||
| api | ||
| doc | ||
| lib/time | ||
| misc | ||
| src | ||
| test | ||
| .gitattributes | ||
| .gitignore | ||
| codereview.cfg | ||
| CONTRIBUTING.md | ||
| go.env | ||
| LICENSE | ||
| PATENTS | ||
| README.md | ||
| SECURITY.md | ||
The Go Programming Language
Go is an open source programming language that makes it easy to build simple, reliable, and efficient software.
Gopher image by Renee French, licensed under Creative Commons 4.0 Attributions license.
Our canonical Git repository is located at https://go.googlesource.com/go. There is a mirror of the repository at https://github.com/golang/go.
Unless otherwise noted, the Go source files are distributed under the BSD-style license found in the LICENSE file.
Download and Install
Binary Distributions
Official binary distributions are available at https://go.dev/dl/.
After downloading a binary release, visit https://go.dev/doc/install for installation instructions.
Install From Source
If a binary distribution is not available for your combination of operating system and architecture, visit https://go.dev/doc/install/source for source installation instructions.
Contributing
Go is the work of thousands of contributors. We appreciate your help!
To contribute, please read the contribution guidelines at https://go.dev/doc/contribute.
Note that the Go project uses the issue tracker for bug reports and proposals only. See https://go.dev/wiki/Questions for a list of places to ask questions about the Go language.