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When a non-Go thread calls into Go, the runtime needs an M to run the Go code. The runtime keeps a list of extra M's available. When the last extra M is allocated, the needextram field is set to tell it to allocate a new extra M as soon as it is running in Go. This ensures that an extra M will always be available for the next thread. However, if many threads need an extra M at the same time, this serializes them all. One thread will get an extra M with the needextram field set. All the other threads will see that there is no M available and will go to sleep. The one thread that succeeded will create a new extra M. One lucky thread will get it. All the other threads will see that there is no M available and will go to sleep. The effect is thundering herd, as all the threads looking for an extra M go through the process one by one. This seems to have a particularly bad effect on the FreeBSD scheduler for some reason. With this change, we track the number of threads waiting for an M, and create all of them as soon as one thread gets through. This still means that all the threads will fight for the lock to pick up the next M. But at least each thread that gets the lock will succeed, instead of going to sleep only to fight again. This smooths out the performance greatly on FreeBSD, reducing the average wall time of `testprogcgo CgoCallbackGC` by 74%. On GNU/Linux the average wall time goes down by 9%. Fixes #13926 Fixes #16396 Change-Id: I6dc42a4156085a7ed4e5334c60b39db8f8ef8fea Reviewed-on: https://go-review.googlesource.com/25047 Run-TryBot: Ian Lance Taylor <iant@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Dmitry Vyukov <dvyukov@google.com> |
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The Go Programming Language
Go is an open source programming language that makes it easy to build simple, reliable, and efficient software.
For documentation about how to install and use Go, visit https://golang.org/ or load doc/install-source.html in your web browser.
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--
Binary Distribution Notes
If you have just untarred a binary Go distribution, you need to set the environment variable $GOROOT to the full path of the go directory (the one containing this file). You can omit the variable if you unpack it into /usr/local/go, or if you rebuild from sources by running all.bash (see doc/install-source.html). You should also add the Go binary directory $GOROOT/bin to your shell's path.
For example, if you extracted the tar file into $HOME/go, you might put the following in your .profile:
export GOROOT=$HOME/go
export PATH=$PATH:$GOROOT/bin
See https://golang.org/doc/install or doc/install.html for more details.