memcpy performance regressions 2.19 -> 2.24(5)

[H.J. Lu]

On Fri, May 12, 2017 at 1:21 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, May 12, 2017 at 12:43 PM, Erich Elsen <eriche@google.com> wrote:
>> HJ - yes, the benchmark still shows the same behavior.  I did have to modify
>> the build to add -std=c++11.
>
> I updated hjl/x86/optimize branch with memcpy_benchmark2.cc
> to change its output for easy comparison.  Please take a look to see
> if it is still valid.
>
> H.J.
>> Carlos - Maybe the first step is to add a tunable that allows for selection
>> of the non-temporal-store size threshold without changing the implementation
>> that is selected.  I can work on submitting this patch.

There are

 /* The large memcpy micro benchmark in glibc shows that 6 times of
     shared cache size is the approximate value above which non-temporal
     store becomes faster.  */
  __x86_shared_non_temporal_threshold = __x86_shared_cache_size * 6;

I did the measurement on a 8-core processor.  6 / 8 is .75 of the shared
cache.   But on processors with 56 cores, 6 / 56 may be too small.

H.J.
>> On Wed, May 10, 2017 at 7:17 PM, Carlos O'Donell <carlos@redhat.com> wrote:
>>>
>>> On 05/10/2017 01:33 PM, H.J. Lu wrote:
>>> > On Tue, May 9, 2017 at 4:48 PM, Erich Elsen <eriche@google.com> wrote:
>>> >> store is a net win even though it causes a 2-3x decrease in single
>>> >> threaded performance for some processors?  Or how else is the decision
>>> >> about the threshold made?
>>> >
>>> > There is no perfect number to make everyone happy.  I am open
>>> > to suggestion to improve the compromise.
>>> >
>>> > H.J.
>>>
>>> I agree with H.J., there is a compromise to be made here. Having a single
>>> process thrash the box by taking all of the memory bandwidth might be
>>> sensible for a microservice, but glibc has to default to something that
>>> works well on average.
>>>
>>> With the new tunables infrastructure we can start talking about ways in
>>> which a tunable could influence IFUNC selection though, allowing users
>>> some kind of choice into tweaking for single-threaded or multi-threaded,
>>> single-user or multi-user etc.
>>>
>>> What I would like to see as the output of any discussion is a set of
>>> microbenchmarks (benchtests/) added to glibc that are the distillation
>>> of whatever workload we're talking about here. This is crucial to the
>>> community having a way to test from release-to-release that we don't
>>> regress performance.
>>>
>>> Unless you want to sign up to test your workload at every release then
>>> we need this kind of microbenchmark addition. And microbenchmarks are
>>> dead-easy to integrate with glibc so most people should have no excuse.
>>>
>>> The hardware vendors and distros who want particular performance tests
>>> are putting such tests in place (representative of their users), and
>>> direct
>>> end-users  who want particular performance are also adding tests.
>>>
>>> --
>>> Cheers,
>>> Carlos.
>>
>>
>
>
>
> --
> H.J.



-- 
H.J.