Date: Wed, 24 Oct 2012 20:06:00 +0100 From: Attilio Rao <attilio@freebsd.org> To: Jim Harris <jim.harris@gmail.com> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, John Baldwin <jhb@freebsd.org> Subject: Re: svn commit: r242014 - head/sys/kern Message-ID: <CAJ-FndDzBdq8q6J7QKqf=abi_702s_ia=pa3XbBv80rxbGb-SA@mail.gmail.com> In-Reply-To: <CAJP=Hc9wLv02sX%2BWnzZtaKccSAFzqg8jT0oP13nLw1jMfwOEBQ@mail.gmail.com> References: <201210241836.q9OIafqo073002@svn.freebsd.org> <201210241443.25988.jhb@freebsd.org> <CAJP=Hc9wLv02sX%2BWnzZtaKccSAFzqg8jT0oP13nLw1jMfwOEBQ@mail.gmail.com>
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On Wed, Oct 24, 2012 at 8:00 PM, Jim Harris <jim.harris@gmail.com> wrote: > On Wed, Oct 24, 2012 at 11:43 AM, John Baldwin <jhb@freebsd.org> wrote: >> On Wednesday, October 24, 2012 2:36:41 pm Jim Harris wrote: >>> Author: jimharris >>> Date: Wed Oct 24 18:36:41 2012 >>> New Revision: 242014 >>> URL: http://svn.freebsd.org/changeset/base/242014 >>> >>> Log: >>> Pad tdq_lock to avoid false sharing with tdq_load and tdq_cpu_idle. >>> >>> This enables CPU searches (which read tdq_load) to operate independently >>> of any contention on the spinlock. Some scheduler-intensive workloads >>> running on an 8C single-socket SNB Xeon show considerable improvement with >>> this change (2-3% perf improvement, 5-6% decrease in CPU util). >>> >>> Sponsored by: Intel >>> Reviewed by: jeff >>> >>> Modified: >>> head/sys/kern/sched_ule.c >>> >>> Modified: head/sys/kern/sched_ule.c >>> >> ============================================================================== >>> --- head/sys/kern/sched_ule.c Wed Oct 24 18:33:44 2012 (r242013) >>> +++ head/sys/kern/sched_ule.c Wed Oct 24 18:36:41 2012 (r242014) >>> @@ -223,8 +223,13 @@ static int sched_idlespinthresh = -1; >>> * locking in sched_pickcpu(); >>> */ >>> struct tdq { >>> - /* Ordered to improve efficiency of cpu_search() and switch(). */ >>> + /* >>> + * Ordered to improve efficiency of cpu_search() and switch(). >>> + * tdq_lock is padded to avoid false sharing with tdq_load and >>> + * tdq_cpu_idle. >>> + */ >>> struct mtx tdq_lock; /* run queue lock. */ >>> + char pad[64 - sizeof(struct mtx)]; >> >> Can this use 'tdq_lock __aligned(CACHE_LINE_SIZE)' instead? >> > > No - that doesn't pad it. I believe that only works if it's global, > i.e. not part of a data structure. As I've already said in another thread __align() doesn't work on object declaration, so what that won't pad it either if it is global or part of a struct. It is just implemented as __attribute__((aligned(X))): http://gcc.gnu.org/onlinedocs/gcc-3.2/gcc/Type-Attributes.html Attilio -- Peace can only be achieved by understanding - A. Einstein
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