[Intel-gfx] [PATCH v2 1/2] i965/gen9: Pass alignment as function parameter in drm_intel_gem_bo_alloc_internal()

[Chris Wilson]

On Tue, Jun 23, 2015 at 04:44:52PM -0700, Anuj Phogat wrote:
> On Mon, Jun 22, 2015 at 1:04 PM, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > On Mon, Jun 22, 2015 at 09:51:08PM +0200, Daniel Vetter wrote:
> >> On Mon, Jun 22, 2015 at 11:47:02AM -0700, Anuj Phogat wrote:
> >> > and use it to initialize the align variable in drm_intel_bo.
> >> >
> >> > In case of YF/YS tiled buffers libdrm need not know about the tiling
> >> > format because these buffers don't have hardware support to be tiled
> >> > or detiled through a fenced region. But, libdrm still need to know
> >> > about buffer alignment restrictions because kernel uses it when
> >> > resolving the relocation.
> >> >
> >> > Mesa uses drm_intel_gem_bo_alloc_for_render() to allocate Yf/Ys buffers.
> >> > So, use the passed alignment value in this function. Note that we continue
> >> > ignoring the alignment value passed to drm_intel_gem_bo_alloc() to follow
> >> > the previous behavior.
> >> >
> >> > V2: Add a condition to avoid allocation from cache. (Ben)
> >>
> >> This will hurt badly since some programs love to cycle through textures.
> >> You can still allocate from the cache, you only need to update the
> >> alignement constraint. The kernel will move the buffer on the next execbuf
> >> if it's misplaced.
> >
> > For llc, using fresh pages just puts memory and aperture pressure (plus
> > a small amount of interface pressure) on the system by allocating more bo.
> >
> > For !llc, it is far better to move an object in the GTT to match a
> > change in alignment than it is to allocate fresh pages (and deallocate
> > stale pages).
> Could you please explain this and point me to what you want to be
> changed in this patch?

You can reuse the cached bo if the alignment mismatches. You can
experiment with searching for a better match, but it's unlikely to be
useful (numbers talk here).

Reusing the cache and keeping the working set small (with zero turnover)
is vital on !llc. Moving a GTT binding is far cheaper than preparing the
backing store for access with the GPU.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre