Re: [PATCH RFC] Improve 64bit memset for Corei7 with avx2 instruction

[Ling Ma <ling dot ma dot program at gmail dot com>]

Ljuba could you please test our patch on haswell with gcc.403 we sent.
I also will test it to compare among, without prfetch, or with
prefetchw and prefetcht0,
gcc.403 benchmark should be more reliable and stringency.

Thanks
Ling



2013/7/30, Ling Ma <ling.ma.program@gmail.com>:
> 2013/7/30, OndÅej BÃlka <neleai@seznam.cz>:
>> On Tue, Jul 30, 2013 at 05:26:09PM +0800, Ling Ma wrote:
>>> We never find prefetcht1 is good instruction to pre-fetch data on
>>> core2, nehalem, sandybridge, and haswell. Our experiments  show
>>> prefetchw is best in your cases.
>>
>> But your code was following:
> Ling: yes, i say we find in your case, prefetchw is the best,
> and we also say we will do further test to verify whether prefetchw is
> better in gcc.403 cases too, if prefetchw is better in gcc.403, we
> will replace prefetcht0 with prefetchw.
>
>>
>> +L(gobble_128_loop):
>> +       prefetcht0      0x1c0(%rdi)
>> +       vmovaps %ymm0, (%rdi)
>> +       prefetcht0      0x280(%rdi)
>> +       vmovaps %ymm0, 0x20(%rdi)
>> +       vmovaps %ymm0, 0x40(%rdi)
>> +       vmovaps %ymm0, 0x60(%rdi)
>> +       lea     0x80(%rdi), %rdi
>> +       sub     $0x80, %rdx
>> +       jae     L(gobble_128_loop)
>>
>> Which uses prefetcht0 (A prefetcht1 in mine benchmark was typo.)
>>
>> I updated benchmark (attached) with your code with and without
>> prefetching.
>> 1)
>>
>> Ljuba could you test it on haswell?
> Ling: Ljuba, please also append with prefetchw, thanks.
>>
>>> In your code, memset only handle 256 bytes, in this case we don't need
>>> to use prefetch because hardware prefetch is enough for us in small
>>> size, but it can tell us whether prefetch will hurt performance so we
>>
>> Does haswell improved hardware prefetcher to fetch from next page? I
>> changed layout of benchmark so that data ends at page boundary.
> Ling: we use software prefetch is becuase it have longer stride than
> hw prefetch,
> so it is good for bigger size.
>>
>>> run it, result is below, it indicates prefetchw on haswell is
>>> harmless, even it is redundant code in memset on haswell.
>>>
>> Your test was invalid as you did compared apples with oranges
>> (prefetcht0 vs prefetchw) To see how your code fares you should replace
>> it with your implementation with and without prefetch.
>> You need that to be exactly what you submitted and if that means
>> prefetchw then post new version.
>>
>>> Then we modified memset2 to handle 4096 bytes
>>> in test.c as bellow
>>> ...
>>> char ary[SIZE+4096];
>>> ...
>>> memset2(ary+(512*((unsigned)rand_r(&seed)))%SIZE,0,4096);
>>> and run your code on haswell as below, result shows prefetchw get
>>> better  performance
>>> and harmless
>>
>> With that 'improvement' you defeated purpose of benchmark. It
>> demonstrated increase cache usage by touching only first half of data
>> and having second half fetched by prefetch.
>>
>> As you only changed size a writes now overlap and there will be no
>> extra memory usage.
>>
>> Also changing it to 4096 decreases percentage of wasted memory. Before
>> it was 50% (256 saved/512 fetched) now its around 11% (4096 saved/ 4608
>> fetched)
>>
>