Quoting Tvrtko Ursulin (2020-12-14 15:57:59)
>
> On 14/12/2020 10:51, Chris Wilson wrote:
> > Refactor the allocation such that we utilise just enough memory pressure
> > to invoke the shrinker, and just enough processes to spread across the
> > CPUs and contend on the shrinker.
> >
> > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> > ---
> > tests/i915/gem_shrink.c | 11 ++++++-----
> > 1 file changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/tests/i915/gem_shrink.c b/tests/i915/gem_shrink.c
> > index 023db8c56..e8a814fe6 100644
> > --- a/tests/i915/gem_shrink.c
> > +++ b/tests/i915/gem_shrink.c
> > @@ -426,6 +426,7 @@ igt_main
> > int num_processes = 0;
> >
> > igt_fixture {
> > + const int ncpus = sysconf(_SC_NPROCESSORS_ONLN);
> > uint64_t mem_size = intel_get_total_ram_mb();
> > int fd;
> >
> > @@ -434,16 +435,16 @@ igt_main
> >
> > /*
> > * Spawn enough processes to use all memory, but each only
> > - * uses half the available mappable aperture ~128MiB.
> > + * uses half of the available per-cpu memory.
> > * Individually the processes would be ok, but en masse
> > * we expect the shrinker to start purging objects,
> > * and possibly fail.
> > */
> > - alloc_size = gem_mappable_aperture_size(fd) / 2;
> > - num_processes = 1 + (mem_size / (alloc_size >> 20));
> > + alloc_size = (mem_size + ncpus - 1) / ncpus / 2;
>
> Div round up with thousands divided by small integers okay, safe on very
> old smp boxes. :)
>
> > + num_processes = ncpus + (mem_size / alloc_size);
>
> Hm, now what does this add up to..
>
> ncpus + mem_size / (mem_size / ncpus / 2) = ... ?
(ncpus + mem_size / (mem_size / ncpus / 2)) * (mem_size / ncpus / 2)
mem_size / 2 + mem_size
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