Re: [PATCH 4/6] drm/i915: Set our shrinker->batch to 4096 (~16MiB)

[Joonas Lahtinen <joonas.lahtinen@linux.intel.com>]

On Tue, 2017-09-26 at 16:02 +0100, Chris Wilson wrote:
> Quoting Joonas Lahtinen (2017-09-20 14:28:40)
> > On Wed, 2017-08-16 at 14:55 +0100, Chris Wilson wrote:
> > > Quoting Joonas Lahtinen (2017-08-16 14:39:00)
> > > > On Sat, 2017-08-12 at 12:51 +0100, Chris Wilson wrote:
> > > > > Prefer to defer activating our GEM shrinker until we have a few
> > > > > megabytes to free; or we have accumulated sufficient mempressure by
> > > > > deferring the reclaim to force a shrink. The intent is that because our
> > > > > objects may typically be large, we are too effective at shrinking and
> > > > > are not rewarded for freeing more pages than the batch. It will also
> > > > > defer the initial shrinking to hopefully put it at a lower priority than
> > > > > say the buffer cache (although it will balance out over a number of
> > > > > reclaims, with GEM being more bursty).
> > > > > 
> > > > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > > > ---
> > > > >  drivers/gpu/drm/i915/i915_gem_shrinker.c | 1 +
> > > > >  1 file changed, 1 insertion(+)
> > > > > 
> > > > > diff --git a/drivers/gpu/drm/i915/i915_gem_shrinker.c b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> > > > > index 5b8bc0e4f336..8bb17e9a52de 100644
> > > > > --- a/drivers/gpu/drm/i915/i915_gem_shrinker.c
> > > > > +++ b/drivers/gpu/drm/i915/i915_gem_shrinker.c
> > > > > @@ -461,6 +461,7 @@ void i915_gem_shrinker_init(struct drm_i915_private *dev_priv)
> > > > >       dev_priv->mm.shrinker.scan_objects = i915_gem_shrinker_scan;
> > > > >       dev_priv->mm.shrinker.count_objects = i915_gem_shrinker_count;
> > > > >       dev_priv->mm.shrinker.seeks = DEFAULT_SEEKS;
> > > > > +     dev_priv->mm.shrinker.batch = 4096;
> > > > 
> > > > Did you try how this alone effects the runtime of two consequtive
> > > > gem.testlist runs? Is there some specific test/usecase that benefits
> > > > from this. We'd be the first one to set this, md/raid5.c sets it to 128
> > > > which is the default (0).
> > > 
> > > My testing was trying to play a game that was hitting swap on an old
> > > hdd. So not very quantifiable, and vmscan is very unintuitive. 
> > > 
> > > Note also that we are special in that we don't report objects but pages.
> > > Not that it makes any difference, upon reclaim every slab is basically
> > > asked to give up some %% of what it reports, with some hysteresis thrown
> > > in on top.
> > > 
> > > The only way we can do anything here is to throw it at lots of systems
> > > and see how that helps. My gut feeling says that the batch size should
> > > be approximately the typical object size in the freeable list, to try to
> > > reduce the amount of inefficient work. Now, the value is read before
> > > scan->count is called, but we can always improve the estimate for the
> > > next pass.
> > 
> > For documentation purposes, from IRC, this is;
> > 
> > Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> 
> And for reference, I took a stab at measuring vmpressure with gem_syslatency
> https://patchwork.freedesktop.org/patch/178777/

And, did we get any results? :)

Regards, Joonas
-- 
Joonas Lahtinen
Open Source Technology Center
Intel Corporation
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