[Bug 80705] [IVB/HSW/BYT-M/BDW] GpuTest_v0.5_triangle performance slower by 10%~50% in fullscreen than in windowed & composited mode

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=80705

--- Comment #4 from Alejandro Piñeiro (freenode IRC: apinheiro) <apinheiro at igalia.com> ---
Hi, these days I have been taking a look to this bug, and I will share my first
discoveries.

(In reply to zhoujian from comment #0)
> 
> Bug detailed description:
> ----------------------------------------------
> GpuTest_v0.5_triangle is slower by 10%~50% in fullscreen than in windowed &
> composited mode on BYT-M/IVB/HSW/BDW, The problem exists both on
> gnome-session and Raw X.

At this moment I was not able to find v0.5, but the bug is reproducible with
the last version v0.7

I detected a ~10% performance lost with 1920x1080 resolution. ~50% using
1280x720

(In reply to Eero Tamminen from comment #1)
> According to Mengmeng, this issue happens only with DRI3, not DRI2.

I reproduced this bug on DRI2. In fact, I was not able to test this on DRI3
after a system upgrade. But I really think that that is unrelated to that bug.

> Can you check with intel_gpu_top (on BYT where it works fine) what is the
> "render" value both in fullscreen and windowed mode?
> 
> I.e. is GPU 100% utilized in both cases, like it's with DRI2.

As zhoujian mentioned on comment 2, GPU is at 100% on both cases.

> FYI: In an unrelated test it was noticed that with DRI3, X still does
> occasionally Vsync wait although test had zeroed swap interval.
> 
> According to Chris' comment on that test:
> ---------
> The current implementation requires 4 buffers to run the entire pipeline in
> an synchronous manner, Keith is only feeding in 3 (from
> mesa/src/glx/dri3_glx.c) and so forces the synchronization.
> ---------
> 
> -> With Vsync, test wouldn't be using GPU fully.

If Vsync were happening with fullscreen, the FPS (that is the measure of
performance in this test) would be equal to the refresh rate. So for example,
in my case 60FPS. But that is not the case, but ~360 FPS. The curious thing is
that it doesn't matter the resolution, on fullscreen the FPS are always 360FPS.
But on windowed I detected ~380 for 1920x1280, ~860 for 1280x720 and so on.

After that I created a small example in order to discard the original test. You
can find it on comment 3, and Im removing the vsync using vblank_mode=0.

Fullscreen:
  1920x1080: ~372
  1280x720:  ~372
  1024x640:  ~372

Windowed
  1920x1080: ~390 
  1280x720:  ~840
  1024x640:  ~1280

So although it is not vsynced, for some reason on fullscreen is set to a
defined FPS.

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