Messages in this thread | | | From | "Huang\, Ying" <> | Subject | Re: [LKP] [lkp] [x86/hweight] 65ea11ec6a: will-it-scale.per_process_ops 9.3% improvement | Date | Wed, 17 Aug 2016 15:29:04 -0700 |
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Borislav Petkov <bp@suse.de> writes:
> On Tue, Aug 16, 2016 at 04:09:19PM -0700, H. Peter Anvin wrote: >> On August 16, 2016 10:16:35 AM PDT, Borislav Petkov <bp@suse.de> wrote: >> >On Tue, Aug 16, 2016 at 09:59:00AM -0700, H. Peter Anvin wrote: >> >> Dang... >> > >> >Isn't 9.3% improvement a good thing(tm) ? >> >> Yes, it's huge. The only explanation I could imagine is that scrambling %rdi caused the scheduler to do completely the wrong thing. > > I'm questioning the validity, actually. Report says test machine was > Sandy Bridge-EP and I'd bet good money this one has POPCNT support so > how are we even hitting that __sw_hweight64() path, at all?
We done 8 tests for the base and 4 tests for the head, and the result is quite stable.
I found there is another change between the two comments,
base:
"perf-stat.branch-miss-rate": [ 0.3089533646503185, 0.3099821038600304, 0.3123762964028104, 0.311511881793534, 0.31231973343587144, 0.3096478429327263, 0.31166037272389924, 0.3097364392684626 ],
first bad commit:
"perf-stat.branch-miss-rate": [ 0.039853905034485354, 0.0402472142423231, 0.04380682345704418, 0.04319082390667179 ],
branch-miss-rate decreased from ~0.30% to ~0.043%.
So I guess there are some code alignment change, which caused decreased branch miss rate.
Best Regards, Huang, Ying
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