[Mesa-dev] Plumbing explicit synchronization through the Linux ecosystem

[Michel Dänzer]

On 2020-03-16 4:50 a.m., Marek Olšák wrote:
> The synchronization works because the Mesa driver waits for idle (drains
> the GFX pipeline) at the end of command buffers and there is only 1
> graphics queue, so everything is ordered.
> 
> The GFX pipeline runs asynchronously to the command buffer, meaning the
> command buffer only starts draws and doesn't wait for completion. If the
> Mesa driver didn't wait at the end of the command buffer, the command
> buffer would finish and a different process could start execution of its
> own command buffer while shaders of the previous process are still running.
> 
> If the Mesa driver submits a command buffer internally (because it's full),
> it doesn't wait, so the GFX pipeline doesn't notice that a command buffer
> ended and a new one started.
> 
> The waiting at the end of command buffers happens only when the flush is
> external (Swap buffers, glFlush).
> 
> It's a performance problem, because the GFX queue is blocked until the GFX
> pipeline is drained at the end of every frame at least.
> 
> So explicit fences for SwapBuffers would help.

Not sure what difference it would make, since the same thing needs to be
done for explicit fences as well, doesn't it?


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer