Re: [PATCH 09/13] drm/i915: don't use gtt_pwrite on LLC cached objects

Daniel Vetter
2011-11-06T22:19:09Z

On Sun, Nov 06, 2011 at 09:16:00PM +0000, Chris Wilson wrote:
> On Sun, 6 Nov 2011 20:13:56 +0100, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> > ~120 µs instead fo ~210 µs to write 1mb on my snb. I like this.
> >
> > Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> > ---
> > drivers/gpu/drm/i915/i915_gem.c | 1 +
> > 1 files changed, 1 insertions(+), 0 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 0048917..8fd175c 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -842,6 +842,7 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
> > ret = i915_gem_phys_pwrite(dev, obj, args, file);
> > goto out;
> > } else if (obj->gtt_space &&
> > + obj->cache_level == I915_CACHE_NONE &&
> > obj->base.write_domain != I915_GEM_DOMAIN_CPU) {
> > ret = i915_gem_object_pin(obj, 0, true);
> > if (ret)
>
> I still think you want to include a obj->map_and_fenceable test here.
> When doing 2D benchmarks the stall incurred here to evict an old object
> map the to-be-written object into the mappable GTT causes measureable
> pain (obviously on non-LLC architectures).

That's one of "further tricks". I think we need to also implement the same
in-place clflush trick like for pread, too, to avoid penalizing partial
pwrites too much.

The other trick is to do reloc fixups through llc/clflushed cpu writes.
This way we'd completely eliminate mappable pressure for all untiled
objects. The only thing left would be scanout, tiled gtt uploads and tiled
blts (only on pre-gen4).
-Daniel
--
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48