On 08/29/2013 11:54 PM, Linus Torvalds wrote:
On Thu, Aug 29, 2013 at 8:12 PM, Waiman Long<waiman.long@xxxxxx> wrote:
On 08/29/2013 07:42 PM, Linus Torvalds wrote:
Waiman? Mind looking at this and testing? Linus
Sure, I will try out the patch tomorrow morning and see how it works out for
my test case.
Ok, thanks, please use this slightly updated pCMPXCHG_LOOPatch attached here.
I tested your patch on a 2-socket (12 cores, 24 threads) DL380 with
2.9GHz Westmere-EX CPUs, the test results of your test program (max
threads increased to 24 to match the thread count) were:
with patch = 68M
w/o patch = 12M
So it was an almost 6X improvement. I think that is really good. A
dual-socket machine, these days, shouldn't be considered as a "BIG"
machine. They are pretty common in different organizations.
I have reviewed the patch, and it looks good to me with the exception
that I added a cpu_relax() call at the end of while loop in the
CMPXCHG_LOOP macro.
I also got the perf data of the test runs with and without the patch.
With patch:
29.24% a.out [kernel.kallsyms] [k] lockref_get_or_lock
19.65% a.out [kernel.kallsyms] [k] lockref_put_or_lock
14.11% a.out [kernel.kallsyms] [k] dput
5.37% a.out [kernel.kallsyms] [k] __d_lookup_rcu
5.29% a.out [kernel.kallsyms] [k] lg_local_lock
4.59% a.out [kernel.kallsyms] [k] d_rcu_to_refcount
:
0.13% a.out [kernel.kallsyms] [k] complete_walk
:
0.01% a.out [kernel.kallsyms] [k] _raw_spin_lock
Without patch:
93.50% a.out [kernel.kallsyms] [k] _raw_spin_lock
0.96% a.out [kernel.kallsyms] [k] dput
0.80% a.out [kernel.kallsyms] [k] kmem_cache_free
0.75% a.out [kernel.kallsyms] [k] lg_local_lock
0.48% a.out [kernel.kallsyms] [k] complete_walk
0.45% a.out [kernel.kallsyms] [k] __d_lookup_rcu
For the other test cases that I am interested in, like the AIM7
benchmark, your patch may not be as good as my original one. I got 1-3M
JPM (varied quite a lot in different runs) in the short workloads on a
80-core system. My original one got 6M JPM. However, the test was done
on 3.10 based kernel. So I need to do more test to see if that has an
effect on the JPM results.
Anyway, I think this patch is good performance-wise. I remembered that
awhile ago that an internal reported a lock contention problem in dentry
involving probably complete_walk(). This patch will certainly help for
that case.
I will do more investigation to see how to make this patch work better
for my test cases.
Thank for taking the effort in optimizing the complete_walk() and
unlazy_walk() function that are not in my original patch. That will make
the patch work even better under more circumstances. I really appreciate
that.
Best regards,
Longman
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