[cfarm-users] Setting up GitLab CI for git.git on the farm

[Segher Boessenkool]

On Sun, Nov 25, 2018 at 10:23:41AM +0100, Baptiste Jonglez wrote:
> On 24-11-18, Segher Boessenkool via cfarm-users wrote:
> > On Sat, Nov 24, 2018 at 11:28:58PM +0100, Ævar Arnfjörð Bjarmason via cfarm-users wrote:
> > > I've tried to be conservative about resources. It's all nice -n 19'd,
> > > and with a conservative -j value relative to the number of cores:
> > > https://gitlab.com/git-vcs/git-gitlab-ci/blob/b8d4645891aa/ci/gitlab/run-on-gcc-farm.sh#L62-163
> > 
> > At least for the Power machines, that isn't conservative at all.
> > -j1 is conservative.  -j24 is not conservative on a machine with 20 CPUs
> > (gcc112), or 32 CPUs (gcc135).  The AIX (gcc119) jobs seem to run for over
> > an hour on half the machine?  That's no good :-(
> 
> According to ansible [https://cfarm.tetaneutral.net/machines/list/] gcc112
> has 160 cores, and gcc135 has 128 cores.  Is ansible getting this wrong?

Yes.
gcc110 is a Power7 with 16 cores, 64 threads.
gcc112 is a Power8 with 20 cores, 160 threads.
gcc135 is a Power9 with 32 cores, 128 threads.

(Running more than 4 threads per core on a Power8 does not really help,
so you can count gcc112 as 80 threads).

Running many tasks using SMT hurts tasks that want to run single-threaded,
of course.  Linux is quite good at distributing things nicely, but
oversubscription hurts.

Automated tasks on shared systems should try to stay out of the way.


Segher