[Mesa-dev] [PATCH] st/dri: decrease input lag by syncing sooner in SwapBuffers

[Marek Olšák]

On Fri, Apr 26, 2019 at 12:56 PM Axel Davy <davyaxel0 at gmail.com> wrote:

> On 26/04/2019 10:08, Michel Dänzer wrote:
> > On 2019-04-26 4:06 a.m., Marek Olšák wrote:
> >> From: Marek Olšák <marek.olsak at amd.com>
> >>
> >> It's done by:
> >> - decrease the number of frames in flight by 1
> >> - flush before throttling in SwapBuffers
> >>    (instead of wait-then-flush, do flush-then-wait)
> >>
> >> The improvement is apparent with Unigine Heaven.
> >>
> >> Previously:
> >>      draw frame 2
> >>      wait frame 0
> >>      flush frame 2
> >>      present frame 2
> >>
> >>      The input lag is 2 frames.
> >>
> >> Now:
> >>      draw frame 2
> >>      flush frame 2
> >>      wait frame 1
> >>      present frame 2
> >>
> >>      The input lag is 1 frame. Flushing is done before waiting, because
> >>      otherwise the device would be idle after waiting.
> > Nice idea. Not sure offhand about all ramifications, but certainly worth
> > a go.
> >
> >
> >> Nine is affected because it also uses the pipe cap.
> >> ---
> >>   src/gallium/auxiliary/util/u_screen.c         |  2 +-
> >>   src/gallium/state_trackers/dri/dri_drawable.c | 20 +++++++++----------
> >>   2 files changed, 11 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/src/gallium/auxiliary/util/u_screen.c
> b/src/gallium/auxiliary/util/u_screen.c
> >> index 27f51e0898e..410f17421e6 100644
> >> --- a/src/gallium/auxiliary/util/u_screen.c
> >> +++ b/src/gallium/auxiliary/util/u_screen.c
> >> @@ -349,21 +349,21 @@ u_pipe_screen_get_param_defaults(struct
> pipe_screen *pscreen,
> >>      case PIPE_CAP_MAX_VARYINGS:
> >>         return 8;
> >>
> >>      case PIPE_CAP_COMPUTE_GRID_INFO_LAST_BLOCK:
> >>         return 0;
> >>
> >>      case PIPE_CAP_COMPUTE_SHADER_DERIVATIVES:
> >>         return 0;
> >>
> >>      case PIPE_CAP_MAX_FRAMES_IN_FLIGHT:
> >> -      return 2;
> >> +      return 1;
> > This might be slightly misleading, as there can still be two frames in
> > flight (on the GPU) at the same time. Might be better to leave this at 2
> > (so Nine isn't affected) and adjust its treatment in
> > src/gallium/state_trackers/dri/dri_drawable.c .
> >
> >
> Checking what gallium nine does currently, it seems we already do flush
> then wait,
> however we call swap_fences_pop_front and swap_fences_push_back in the
> reverse order compared to your patch.
> We compensate by taking PIPE_CAP_MAX_FRAMES_IN_FLIGHT + 1
>
> In conclusion, with the proposed patch, gl and nine should have the same
> behaviour (and thus if gl benefits from a value of 1, nine should as well).
> I haven't have noticed input lag, I guess I have to test on heaven if
> you see a difference.
> How can I slow down my gpu to test that ? I use to use the
> /sys/kernel/debug/dri/0/ vars to force low dpm, but it doesn't seem to
> be possible anymore as the related files are gone (rx480) ?
>

I set maximum settings, windowed, resolution: custom, and I type in the 4K
resolution (I don't have a 4K monitor). When it's running, I enable
wireframe. It should be pretty slow.

Marek
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