Date: Thu, 19 Jan 2012 11:24:12 +0800 From: David Xu <listlog2011@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, davidxu@freebsd.org Subject: Re: svn commit: r230201 - head/lib/libc/gen Message-ID: <4F178CDC.3030807@gmail.com> In-Reply-To: <201201181009.23221.jhb@freebsd.org> References: <201201160615.q0G6FE9r019542@svn.freebsd.org> <201201170957.47718.jhb@freebsd.org> <4F1629D5.4020605@gmail.com> <201201181009.23221.jhb@freebsd.org>
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On 2012/1/18 23:09, John Baldwin wrote: > On Tuesday, January 17, 2012 9:09:25 pm David Xu wrote: >> On 2012/1/17 22:57, John Baldwin wrote: >>> On Monday, January 16, 2012 1:15:14 am David Xu wrote: >>>> Author: davidxu >>>> Date: Mon Jan 16 06:15:14 2012 >>>> New Revision: 230201 >>>> URL: http://svn.freebsd.org/changeset/base/230201 >>>> >>>> Log: >>>> Insert read memory barriers. >>> I think using atomic_load_acq() on sem->nwaiters would be clearer as it would >>> indicate which variable you need to ensure is read after other operations. In >>> general I think raw rmb/wmb usage should be avoided when possible as it is >>> does not describe the programmer's intent as well. >>> >> Yes, I had considered that I may use atomic_load_acq(), but at that time, >> I thought it emits a bus locking, right ? so I just picked up rmb() which >> only affects current cpu. maybe atomic_load_acq() does same thing with >> rmb() ? >> it is still unclear to me. > atomic_load_acq() is the same as rmb(). Right now it uses a locked > instruction on amd64, but it could easily switch to lfence/sfence instead. I > had patches to do that but I think bde@ had done some benchmarks that showed > that change made no difference. > I wish there is a version uses lfence for atomic_load_acq(). I always think bus locking is expensive on a multiple-core machine. Here we work on large machine found that even current rwlock in libthr is not scale well if most threads are readers, we have to implement CSNZI-like rwlock to avoid CPU conflict. http://people.csail.mit.edu/mareko/spaa09-scalablerwlocks.pdf I have just done a benchmark on my notebook which is a 4 SMT sandy bridge CPU i3 2310m. http://people.freebsd.org/~davidxu/bench/semaphore/ <http://people.freebsd.org/%7Edavidxu/bench/semaphore/>; The load_acq uses atomic locking is much slower than lfence: http://people.freebsd.org/~davidxu/bench/semaphore/ministat.txt <http://people.freebsd.org/%7Edavidxu/bench/semaphore/ministat.txt>; benchmark program: http://people.freebsd.org/~davidxu/bench/semaphore/sem_test.c <http://people.freebsd.org/%7Edavidxu/bench/semaphore/sem_test.c>; Regards, David Xu
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