On Wed, 28 Nov 2012 10:14:44 +1100, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> + * XXX: (dchinner) This is one of the worst cases of shrinker abuse I've
> + *
> + * 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,
> + * the minimum object size is a page.
> + *
> + * But the craziest part comes when i915_gem_purge() has walked all the
> + * 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
> + * hammer because it was busy doing stuff when something else generated
> + * 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 Wilson, Intel Open Source Technology Centre