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
