Date: Wed, 24 Oct 2012 22:25:03 +0200 From: Andre Oppermann <andre@freebsd.org> To: attilio@FreeBSD.org Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Jim Harris <jim.harris@gmail.com>, John Baldwin <jhb@freebsd.org> Subject: Re: svn commit: r242014 - head/sys/kern Message-ID: <50884E9F.3090706@freebsd.org> In-Reply-To: <CAJ-FndDzBdq8q6J7QKqf=abi_702s_ia=pa3XbBv80rxbGb-SA@mail.gmail.com> References: <201210241836.q9OIafqo073002@svn.freebsd.org> <201210241443.25988.jhb@freebsd.org> <CAJP=Hc9wLv02sX%2BWnzZtaKccSAFzqg8jT0oP13nLw1jMfwOEBQ@mail.gmail.com> <CAJ-FndDzBdq8q6J7QKqf=abi_702s_ia=pa3XbBv80rxbGb-SA@mail.gmail.com>
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On 24.10.2012 21:06, Attilio Rao wrote: > 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 Actually it seems gcc itself doesn't really care and it up to the linker to honor that. -- Andre
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