Re: memcpy performance regressions 2.19 -> 2.24(5)

["H.J. Lu" <hjl.tools@gmail.com>]

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.
>
> 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.