Re: [PATCH] ARM64: Improve copy_page for 128 cache line sizes.

[Andrew Pinski]

On Tue, Dec 21, 2015 at 5:43 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Monday 21 December 2015, Will Deacon wrote:
>> On Sat, Dec 19, 2015 at 04:11:18PM -0800, Andrew Pinski wrote:
>> > Adding a check for the cache line size is not much overhead.
>> > Special case 128 byte cache line size.
>> > This improves copy_page by 85% on ThunderX compared to the original
>> > implementation.
>>
>> So this patch seems to:
>>
>>   - Align the loop
>>   - Increase the prefetch size
>>   - Unroll the loop once
>>
>> Do you know where your 85% boost comes from between these? I'd really
>> like to avoid having multiple versions of copy_page, if possible, but
>> maybe we could end up with something that works well enough regardless
>> of cacheline size. Understanding what your bottleneck is would help to
>> lead us in the right direction.

I think it is the prefetching.  ThunderX T88 pass 1 and pass 2 does
not have a hardware prefetcher so prefetching a half of a cacheline
ahead does not help at all.

>>
>> Also, how are you measuring the improvement? If you can share your
>> test somewhere, I can see how it affects the other systems I have
>> access to.

You can find my benchmark at
https://github.com/apinski-cavium/copy_page_benchmark .
copy_page is my previous patch.
copy_page128 is just the unrolled and only 128 byte prefetching
copy_page64 is the original code
copy_page64unroll is the new patch which I will be sending out soon.

>
> A related question would be how other CPU cores are affected by the change.
> The test for the cache line size is going to take a few cycles, possibly a lot on certain implementations, e.g. if we ever get one where 'mrs' is microcoded or trapped by a hypervisor.
>
> Are there any possible downsides to using the ThunderX version on other microarchitectures too and skip the check?

Yes that is a good idea.  I will send out a new patch in a little bit
which just unrolls the loop with keeping of the two prefetch
instructions in there.

Thanks,
Andrew Pinski

>
>         Arnd
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