linux-kernel - Re: x86 memcpy performance

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Date:	Thu, 01 Sep 2011 17:15:22 +0200
From:	Maarten Lankhorst <m.b.lankhorst@...il.com>
To:	Borislav Petkov <bp@...64.org>
CC:	"Valdis.Kletnieks@...edu" <Valdis.Kletnieks@...edu>,
	Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...e.hu>,
	melwyn lobo <linux.melwyn@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: x86 memcpy performance

Hey,

2011/8/16 Borislav Petkov <bp@...64.org>:
> On Mon, Aug 15, 2011 at 10:34:35PM -0400, Valdis.Kletnieks@...edu wrote:
>> On Sun, 14 Aug 2011 11:59:10 +0200, Borislav Petkov said:
>>
>> > Benchmarking with 10000 iterations, average results:
>> > size    XM              MM              speedup
>> > 119     540.58          449.491         0.8314969419
>>
>> > 12273   2307.86         4042.88         1.751787902
>> > 13924   2431.8          4224.48         1.737184756
>> > 14335   2469.4          4218.82         1.708440514
>> > 15018 2675.67         1904.07         0.711622886
>> > 16374   2989.75         5296.26         1.771470902
>> > 24564   4262.15         7696.86         1.805863077
>> > 27852   4362.53         3347.72         0.7673805572
>> > 28672   5122.8          7113.14         1.388524413
>> > 30033   4874.62         8740.04         1.792967931
>>
>> The numbers for 15018 and 27852 are *way* odd for the MM case. I don't feel
>> really good about this till we understand what happened for those two cases.
>
> Yep.
>
>> Also, anytime I see "10000 iterations", I ask myself if the benchmark
>> rigging took proper note of hot/cold cache issues. That *may* explain
>> the two oddball results we see above - but not knowing more about how
>> it was benched, it's hard to say.
>
> Yeah, the more scrutiny this gets the better. So I've cleaned up my
> setup and have attached it.
>
> xm_mem.c does the benchmarking and in bench_memcpy() there's the
> sse_memcpy call which is the SSE memcpy implementation using inline asm.
> It looks like gcc produces pretty crappy code here because if I replace
> the sse_memcpy call with xm_memcpy() from xm_memcpy.S - this is the
> same function but in pure asm - I get much better numbers, sometimes
> even over 2x. It all depends on the alignment of the buffers though.
> Also, those numbers don't include the context saving/restoring which the
> kernel does for us.
>
> 7491    1509.89         2346.94         1.554378381
> 8170    2166.81         2857.78         1.318890326
> 12277   2659.03         4179.31         1.571744176
> 13907   2571.24         4125.7          1.604558427
> 14319   2638.74         5799.67         2.19789466      <----
> 14993   2752.42         4413.85         1.603625603
> 16371   3479.11         5562.65         1.59887055

This work intrigued me, in some cases kernel memcpy was a lot faster than sse memcpy,
and I finally figured out why. I also extended the test to an optimized avx memcpy,
but I think the kernel memcpy will always win in the aligned case.

Those numbers you posted aren't right it seems. It depends a lot on the alignment,
for example if both are aligned to 64 relative to each other,
kernel memcpy will win from avx memcpy on my machine.

I replaced the malloc calls with memalign(65536, size + 256) so I could toy
around with the alignments a little. This explains why for some sizes, kernel
memcpy was faster than sse memcpy in the test results you had.
When (src & 63 == dst & 63), it seems that kernel memcpy always wins, otherwise
avx memcpy might.

If you want to speed up memcpy, I think your best bet is to find out why it's
so much slower when src and dst aren't 64-byte aligned compared to each other.

Cheers,
Maarten

---
Attached: my modified version of the sse memcpy you posted.

I changed it a bit, and used avx, but some of the other changes might
be better for your sse memcpy too.

View attachment "ym_memcpy.txt" of type "text/plain" (2668 bytes)