[Mesa-dev] [PATCH 0/7] st/mesa: glReadPixels cache

[Brian Paul]

On 06/15/2016 02:38 AM, Nicolai Hähnle wrote:
> Hi,
>
> some applications use successive calls to glReadPixels to read data back.
> This typically involves a GPU-based blit for each call for de-tiling or
> format conversions (e.g. BGRA -> RGBA). Even when the _mesa_readpixels path
> is used, such a blit tends to be hidden behind the transfer operations. The
> overhead is rather bad for performance, since we have to wait for GPU idle
> each time.
>
> This patch series implements a cache which heuristically does a blit of the
> entire framebuffer into a temporary texture once, which is then re-used by
> immediately following calls to glReadPixels. The cache remains disabled for
> drivers that do not prefer blit based texture transfers, i.e. softpipe/llvmpipe.
>
> Aside from a client's application, this also affects ~1400 piglit tests,
> which tend to see speedups of 5-10% with this cache in my tests.
>
> While looking for places that invalidate the cache, I also noticed a few
> additional spots where I believe the bitmap cache needs to be flushed. I put
> those first in this series.
>
> Please review!

Patches 1-3 look OK to me.  Though, our bitmap cache isn't really 
conformant anyway when texturing is enabled for glBitmap.

I'm worried that this optimization will negatively impact 
llvmpipe/softpipe.  I really don't want llvmpipe piglit runs to be any 
slower.  And are there cases where apps might be slower with the cache 
and HW drivers?

Thoughts?

-Brian


> Thanks,
> Nicolai
> --
>   src/mesa/state_tracker/st_atom_framebuffer.c |   1 +
>   src/mesa/state_tracker/st_cb_bitmap.c        |   3 +
>   src/mesa/state_tracker/st_cb_blit.c          |   1 +
>   src/mesa/state_tracker/st_cb_clear.c         |   1 +
>   src/mesa/state_tracker/st_cb_compute.c       |   4 +
>   src/mesa/state_tracker/st_cb_copyimage.c     |   4 +
>   src/mesa/state_tracker/st_cb_drawpixels.c    |   2 +
>   src/mesa/state_tracker/st_cb_drawtex.c       |   1 +
>   src/mesa/state_tracker/st_cb_fbo.h           |   2 +
>   src/mesa/state_tracker/st_cb_readpixels.c    | 244 +++++++++++++----
>   src/mesa/state_tracker/st_cb_texture.c       |  12 +
>   src/mesa/state_tracker/st_context.c          |   3 +
>   src/mesa/state_tracker/st_context.h          |  11 +
>   src/mesa/state_tracker/st_draw.c             |   1 +
>   src/mesa/state_tracker/st_draw_feedback.c    |   1 +
>   src/mesa/state_tracker/st_gen_mipmap.c       |   4 +
>   16 files changed, 237 insertions(+), 58 deletions(-)
>
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