GEM memory DOS (WAS Re: [PATCH 3/3] drm/ttm: under memory pressure minimize the size of memory pool)

[Daniel Vetter]

On Wed, Aug 13, 2014 at 6:38 PM, Daniel Vetter <daniel at ffwll.ch> wrote:
>> Yes, it currently limits you to half of memory, but at least we would
>> like to make it tuneable since there are a lot of user cases where the
>> user wants to use 90% of memory for GPU tasks at the expense of
>> everything else.
>
> Ime a lot of fun stuff starts to happen when you go there. We have piles
> of memory thrashing testcases and generally had lots of fun with our
> shrinker, so I think until you've really beaten onto those paths in
> ttm+radeon I'd keep the limit where it is.

One example that already starts if you go above 50% is that by default
the dirty pagecache is limited to 40% of memory. Above that you start
to stall in writeback, but gpus are really fast at re-dirtying memory.
So we've seen cases where the core mm OOMed with essentially 90% of
memory on writeback and piles of free swap. Waiting a few seconds for
the SSD to catch up would have gotten it out of that tight spot
without killing any process. One side-effect of such fun is that
memory allocations start to fail in really interesting places, and you
need to pile in hacks so make it all a bit more synchronous to avoid
the core mm freaking out.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch