[Intel-gfx] gem clflush optimization for media encoding

[Chris Wilson]

On Wed, 22 Jun 2011 09:20:35 -0700, Keith Packard <keithp at keithp.com> wrote:
> On Wed, 22 Jun 2011 08:29:24 +0200, Daniel Vetter <daniel at ffwll.ch> wrote:
> 
> > The important thing is that you may never use the cpu mappings with
> > these functions (for objects of similar size). Because libdrm reuses
> > bos without checking their domain, you'll get tons of unnecessary
> > clflush even on objects that do not get accessed through the cpu
> > domain.
> 
> Yeah, the problem is that a BO which is not pinned down may get paged
> out, in which case it lands in the CPU domain. I'm not sure we've ever
> added an optimization to avoid flushing objects which are known not to
> have been written to disk?

I've toyed with such. Can be very effective for large working sets like
firefox thrashing the aperture. A very simple example is
firefox-planet-gnome which demonstrates the effect just by scrolling
within a single page on gen3. [The trace currently spends 35% of its time
in clflush which can be entirely eliminated by such tracking.] There's
also the secondary benefit that shmemfs is quite slow for our purposes.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre