[Bug 111090] 3% perf drop in GfxBench Manhattan 3.0, 3.1 and CarChase test-cases

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=111090

--- Comment #3 from Eero Tamminen <eero.t.tamminen at intel.com> ---
Lakshmi, although you marked this as BXT, there was some impact visible also on
KBL GT3e.  I don't think issue is BXT specific, it's visibility just depends on
relative CPU/GPU/bandwidth balance, not absolute performance, and/or how TDP
limited the device is.


(In reply to Chris Wilson from comment #1)
>     drm/i915/execlists: Minimalistic timeslicing
...
> is at least one intentional change in the scheduling that may interfere with
> throughput tests (like gfxbench) in the presence of third parties (X) also
> trying to render.

All the impacted tests are complex/heavy ones, and they run in fullscreen under
Unity which handles fullscreen properly (unlike Gnome [1]) and disables
compositing.  I.e. there's only single process rendering during these
benchmarks.


(In reply to Chris Wilson from comment #2)
> I would say the biggest potential impact there is:
>       drm/i915: Move GEM object domain management from struct_mutex to local
>       drm/i915: Drop the deferred active reference
> 
> Removing struct_mutex should have been an improvement (reducing lock
> contention, and at worse not making it any worse), so the extra burden of
> active reference tracking?

In case it helps, these tests have hundreds of drawcalls per frame and don't
have a any single bottleneck, bottleneck can change many times during frame. 
Unlike the other benchmarks I'm running, the most impacted Manhattan tests use
UBOs and transform feedback.

What about drm-tip non-i915 code changes from upstream rebases?  Test machines
use P-state power management mode and GPU benchmarks on TDP-limited devices
could be impacted also by non-i915 kernel changes.


[1] https://gitlab.gnome.org/GNOME/mutter/issues/60

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