[Intel-gfx] [PATCH] [v4] drm/i915/hsw: Change default LLC age to 3

[Daniel Vetter]

On Sun, Aug 04, 2013 at 11:47:29PM -0700, Ben Widawsky wrote:
> The default LLC age was changed:
> commit 0d8ff15e9a15f2b393e53337a107b7a1e5919b6d
> Author: Ben Widawsky <benjamin.widawsky at intel.com>
> Date:   Thu Jul 4 11:02:03 2013 -0700
> 
> drm/i915/hsw: Set correct Haswell PTE encodings.
> 
> On the surface it would seem setting a default age wouldn't matter
> because all GEM BOs are aged similarly, so the order in which objects
> are evicted would not be subject to aging. The current working theory as
> to why this caused a regression though is that LLC is a bit special in
> that it is shared with the CPU. Presumably (not verified) the CPU
> fetches cachelines with age 3, and therefore recently cached GPU objects
> would be evicted before similar CPU object first when the LLC is full.
> It stands to reason therefore that this would negatively impact CPU
> bound benchmarks - but those seem to be low on the priority list.
> 
> eLLC OTOH does not have this same property as LLC. It should be used
> entirely for the GPU, and so the age really shouldn't matter.
> Furthermore, we have no evidence to suggest one is better than another
> on eLLC. Since we've never properly supported eLLC before no, there
> should be no regression. If the GPU client really wants "younger"
> objects, they should use MOCS.
> 
> v2: Drop the extra #define (Chad)
> 
> v3: Actually git add
> 
> v4: Pimped commit message
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=67062
> Signed-off-by: Ben Widawsky <ben at bwidawsk.net>
> Reviewed-by: Chad Versace <chad.versace at linux.intel.com>
Queued for -next, thanks for the patch.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch