On 2012/1/19 11:24, David Xu wrote:
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>
rdtsc() may not work on SMP, so I have updated it to use clock_gettime
to get total time.
http://people.freebsd.org/~davidxu/bench/semaphore2/
<http://people.freebsd.org/%7Edavidxu/bench/semaphore2/>
Still, lfence is a lot faster than atomic lock.
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