* Fwd: [PATCH] spinlock spin with pause instruction [not found] <CAKw7uVgrjqZVznRMoCbsjyz4YXast5YtAAmpWQorOw7YXqbOhw@mail.gmail.com> @ 2016-03-11 10:28 ` Václav Haisman 2016-03-11 13:04 ` Corinna Vinschen 2016-03-12 6:25 ` Fwd: " Mark Geisert 0 siblings, 2 replies; 4+ messages in thread From: Václav Haisman @ 2016-03-11 10:28 UTC (permalink / raw) To: cygwin-patches [-- Attachment #1: Type: text/plain, Size: 283 bytes --] Hi. I have noticed that Cygwin's spinlock goes into heavy sleeping code for each spin. It seems it would be a good idea to actually try to spin a bit first. There is this 'pause' instruction which let's the CPU make such busy loops be less busy. Here is a patch to do this. -- VH [-- Attachment #2: spinlock-pause.patch.txt --] [-- Type: text/plain, Size: 1258 bytes --] diff --git a/winsup/cygwin/thread.cc b/winsup/cygwin/thread.cc index 01cfd5b..56e66f1 100644 --- a/winsup/cygwin/thread.cc +++ b/winsup/cygwin/thread.cc @@ -1917,6 +1917,19 @@ pthread_spinlock::lock () { pthread_t self = ::pthread_self (); int result = -1; + unsigned spins = 0; + + /* + We want to spin using 'pause' instruction on multi-core system but we + want to avoid this on single-core systems. + + The limit of 1000 spins is semi-arbitrary. Microsoft suggests (in their + InitializeCriticalSectionAndSpinCount documentation on MSDN) they are + using spin count limit 4000 for their heap manager critical + sections. Other source suggest spin count as small as 200 for fast path + of mutex locking. + */ + unsigned const FAST_SPINS_LIMIT = wincap.cpu_count () != 1 ? 1000 : 0; do { @@ -1925,8 +1938,13 @@ pthread_spinlock::lock () set_owner (self); result = 0; } - else if (pthread::equal (owner, self)) + else if (unlikely(pthread::equal (owner, self))) result = EDEADLK; + else if (spins < FAST_SPINS_LIMIT) + { + ++spins; + __asm__ volatile ("pause":::); + } else { /* Minimal timeout to minimize CPU usage while still spinning. */ ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] spinlock spin with pause instruction 2016-03-11 10:28 ` Fwd: [PATCH] spinlock spin with pause instruction Václav Haisman @ 2016-03-11 13:04 ` Corinna Vinschen 2016-03-12 6:25 ` Fwd: " Mark Geisert 1 sibling, 0 replies; 4+ messages in thread From: Corinna Vinschen @ 2016-03-11 13:04 UTC (permalink / raw) To: cygwin-patches [-- Attachment #1: Type: text/plain, Size: 518 bytes --] On Mar 11 11:28, Václav Haisman wrote: > Hi. > > I have noticed that Cygwin's spinlock goes into heavy sleeping code > for each spin. It seems it would be a good idea to actually try to > spin a bit first. There is this 'pause' instruction which let's the > CPU make such busy loops be less busy. Here is a patch to do this. Applied. Thanks, Corinna -- Corinna Vinschen Please, send mails regarding Cygwin to Cygwin Maintainer cygwin AT cygwin DOT com Red Hat [-- Attachment #2: signature.asc --] [-- Type: application/pgp-signature, Size: 819 bytes --] ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Fwd: [PATCH] spinlock spin with pause instruction 2016-03-11 10:28 ` Fwd: [PATCH] spinlock spin with pause instruction Václav Haisman 2016-03-11 13:04 ` Corinna Vinschen @ 2016-03-12 6:25 ` Mark Geisert 2016-03-12 8:14 ` Václav Haisman 1 sibling, 1 reply; 4+ messages in thread From: Mark Geisert @ 2016-03-12 6:25 UTC (permalink / raw) To: cygwin-patches Václav Haisman wrote: > Hi. > > I have noticed that Cygwin's spinlock goes into heavy sleeping code > for each spin. It seems it would be a good idea to actually try to > spin a bit first. There is this 'pause' instruction which let's the > CPU make such busy loops be less busy. Here is a patch to do this. I wanted to try out this patch but compilation is failing on the "unlikely" call. Is that a macro that needs defining or something else? Thanks, ..mark ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Fwd: [PATCH] spinlock spin with pause instruction 2016-03-12 6:25 ` Fwd: " Mark Geisert @ 2016-03-12 8:14 ` Václav Haisman 0 siblings, 0 replies; 4+ messages in thread From: Václav Haisman @ 2016-03-12 8:14 UTC (permalink / raw) To: cygwin-patches [-- Attachment #1: Type: text/plain, Size: 777 bytes --] On 12.3.2016 07:25, Mark Geisert wrote: > Václav Haisman wrote: >> Hi. >> >> I have noticed that Cygwin's spinlock goes into heavy sleeping code >> for each spin. It seems it would be a good idea to actually try to >> spin a bit first. There is this 'pause' instruction which let's the >> CPU make such busy loops be less busy. Here is a patch to do this. > > I wanted to try out this patch but compilation is failing on the > "unlikely" call. Is that a macro that needs defining or something else? > Thanks, > > ..mark > `unlikely()` is a macro defined in `miscfuncs.h`, right below header guard: <https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=winsup/cygwin/miscfuncs.h;h=3a9f0258c6cbb62b8d51e96c0c5542b70659e17b;hb=HEAD> -- VH [-- Attachment #2: OpenPGP digital signature --] [-- Type: application/pgp-signature, Size: 213 bytes --] ^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2016-03-12 8:14 UTC | newest] Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- [not found] <CAKw7uVgrjqZVznRMoCbsjyz4YXast5YtAAmpWQorOw7YXqbOhw@mail.gmail.com> 2016-03-11 10:28 ` Fwd: [PATCH] spinlock spin with pause instruction Václav Haisman 2016-03-11 13:04 ` Corinna Vinschen 2016-03-12 6:25 ` Fwd: " Mark Geisert 2016-03-12 8:14 ` Václav Haisman
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