On Wed, 28 Nov 2012 10:14:44 +1100, Dave Chinner [off-list ref] 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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