Re: [PATCH] Revert "drm/i915: Kill GTT mappings when moving from GTT domain"

[Chris Wilson <chris@chris-wilson.co.uk>]

On Tue, 14 Jun 2011 16:43:09 -0700, Eric Anholt <eric@anholt.net> wrote:
> This reverts commit 4a684a4117abd756291969336af454e8a958802f.
> Userland has always been required to set the object's domain to GTT
> before using it through a GTT mapping, it's not something that the
> kernel is supposed to enforce.  (The pagefault support is so that we
> can handle multiple mappings without userland having to pin across
> them, not so that userland can use GTT after GPU domains without
> telling the kernel).

The only place that can do accurate per-page tracking of domains is the
kernel. (And avoid clflushes for GTT eviction. Ok, so we're not quite
there yet.) Relying on userspace to get it right, and for co-operating
processes to coordinate their access to buffers is a misfeature.
 
> Fixes 19.2% +/- 0.8% (n=6) performance regression in cairo-gl
> firefox-talos-gfx on my T420 latop.

Since GTT mappings are fundamentally slow, don't you think this suggests
that mesa is doing something wrong? Or a bug in cairo-gl to make mesa act
this way?

For the record, on my SugarBay, this patch on top of drm-intel-fixes and
mesa.git + your instruction streaming, regresses cairo-xlib 4.5s to 4.7s
(4%, with a claim of .1% stddev) and improves cairo-gl from 21.5s to 20.1s
(6.5%, with a claim of .5% stddev) for the firefox-talos-gfx. [This is a
system that can do under 2s for cairo-xlib using vmap and the perf
patches I posted before.]
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre