On Tue., Mar. 17, 2020, 06:02 Michel Dänzer, <mic...@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 <mic...@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 >
_______________________________________________ wayland-devel mailing list wayland-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/wayland-devel
