Plumbing explicit synchronization through the Linux ecosystem

[Marek Olšák]

On Tue., Mar. 17, 2020, 06:02 Michel Dänzer, <michel at daenzer.net> wrote:

> On 2020-03-16 7:33 p.m., Marek Olšák wrote:
> > On Mon, Mar 16, 2020 at 5:57 AM Michel Dänzer <michel at daenzer.net>
> wrote:
> >> 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?
> >
> > No. Explicit fences don't require userspace to wait for idle in the
> command
> > buffer. Fences are signalled when the last draw is complete and caches
> are
> > flushed. Before that happens, any command buffer that is not dependent on
> > the fence can start execution. There is never a need for the GPU to be
> idle
> > if there is enough independent work to do.
>
> I don't think explicit fences in the context of this discussion imply
> using that different fence signalling mechanism though. My understanding
> is that the API proposed by Jason allows implicit fences to be used as
> explicit ones and vice versa, so presumably they have to use the same
> signalling mechanism.
>
>
> Anyway, maybe the different fence signalling mechanism you describe
> could be used by the amdgpu kernel driver in general, then Mesa could
> drop the waits for idle and get the benefits with implicit sync as well?
>

Yes. If there is any waiting, or should be done in the GPU scheduler, not
in the command buffer, so that independent command buffers can use the GFX
queue.

Marek


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