Messages in this thread | | | From | Chris Wilson <> | Subject | Re: [PATCH 17/19] drivers: convert shrinkers to new count/scan API | Date | Wed, 28 Nov 2012 01:13:11 +0000 |
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On Wed, 28 Nov 2012 10:14:44 +1100, Dave Chinner <david@fromorbit.com> wrote: > +/* > + * XXX: (dchinner) This is one of the worst cases of shrinker abuse I've seen. > + * > + * i915_gem_purge() expects a byte count to be passed, and the minimum object > + * size is PAGE_SIZE.
No, purge() expects a count of pages to be freed. Each pass of the shrinker therefore tries to free a minimum of 128 pages.
> The shrinker doesn't work on bytes - it works on > + * *objects*.
And I thought you were reviewing the shrinker API to be useful where a single object may range between 4K and 4G.
> So it passes a nr_to_scan of 128 objects, which is interpreted > + * here to mean "free 128 bytes". That means a single object will be freed, as > + * the minimum object size is a page. > + * > + * But the craziest part comes when i915_gem_purge() has walked all the objects > + * and can't free any memory. That results in i915_gem_shrink_all() being > + * called, which idles the GPU and frees everything the driver has in it's > + * active and inactive lists. It's basically hitting the driver with a great big > + * hammer because it was busy doing stuff when something else generated memory > + * pressure. This doesn't seem particularly wise... > + */
As opposed to triggering an OOM? The choice was between custom code for a hopefully rare code path in a situation of last resort, or first implementing the simplest code that stopped i915 from starving the system of memory. -Chris
-- Chris Wilson, Intel Open Source Technology Centre
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