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

[Erich Elsen <eriche@google.com>]

HJ - yes, the benchmark still shows the same behavior.  I did have to
modify the build to add -std=c++11.

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.