[Bug 108598] [GEN9] 20% perf drop in windowed/composited GpuTest Triangle

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=108598

--- Comment #12 from Chris Wilson <chris at chris-wilson.co.uk> ---
(In reply to Eero Tamminen from comment #11)
> There was a windowed Triangle perf improvement in early December, along with
> large reduction in variance.
> 
> Another clear perf improvement (and variance reduction) happened recently,
> between:
> 2019-01-22 14:34:34 af90b519a6: drm-tip: 2019y-01m-22d-14h-33m-29s UTC
> integration manifest
> 2019-01-23 16:59:31 198addb18e: drm-tip: 2019y-01m-23d-16h-58m-20s UTC
> integration manifest

Only one thing of significance there,

commit 6e062b60b0b1bd82cac475e63cdb8c451647182b
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Wed Jan 23 13:51:55 2019 +0000

    drm/i915/execlists: Mark up priority boost on preemption

which should help prevent some surplus preemption loops and could definitely
have a small, variable impact on performance.

> I think windowed GpuTest Triangle perf is still a bit below what it was
> originally (I need to check that from older data, I don't see it anymore in
> the trends).

There's a tiny bit more work to come tweaking need_preempt() to avoid excess
preempt-to-idle cycles, but it should be diminishing returns.

Confidence is high that preemption was causing at least part of the problem
though.

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