On Thu, Aug 31, 2017 at 11:32 AM, Carlos O'Donell <carlos(a)redhat.com> wrote:
On 08/31/2017 04:29 AM, Florian Weimer wrote:
> I would like to tweak glibc so that it only builds one POWER 620/970
> libc for ppc64, and not multiple libcs with POWER6/POWER7/POWER8
> optimizations.
>
> This does not affect ppc64le in any way.
>
> The reason for this change is that the ppc64 builders are awfully slow
> as of late, and the multiple builds (four times currently) currently
> make ppc64 the slowest architecture to build by quite a margin.
>
> Any comments?
IBM has argued in the past that these POWER6/POWER7/POWER8 variants were
important and brought performance gains that were not attainable with
IFUNCs alone.
Therefore we implemented this using IBM's guidance in Fedora to provide
the best possible performance for our users systems.
Yes. That is partly why Florian is reaching out now, to ensure we
solicit feedback from the relevant community of which IBM is a large
part.
As a middle ground, does disabling POWER6 and POWER7 variants save
significant time? Having 970 and POWER8 covers the lowest commonly
avaiable "cheap" option as well as the newest released POWER hardware.
This problem doesn't go away for ppc64le either, once we have
POWER9
systems, it will be a question of doing POWER9 multilibs also?
Perhaps we come up with a defacto standard of supporting N and N-1 for
the current POWER hardware generations, with the change happening on a
Fedora release boundary? So when POWER9 comes out, we build POWER8
and POWER9?
In summary:
- Is the added performance of the multilib builds for users worth the
build cost and development slowdown caused by slow builds?
That is indeed a good question overall. If the answer is no, then all
of the above is irrelevant. If the answer is yes, which I suspect it
to be, then data showing that would be good to have.
josh