[Intel-gfx] [PATCH] drm/i915: Do not use iowait while waiting for the GPU

[Chris Wilson]

Quoting Francisco Jerez (2018-07-28 06:20:12)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > A recent trend for cpufreq is to boost the CPU frequencies for
> > iowaiters, in particularly to benefit high frequency I/O. We do the same
> > and boost the GPU clocks to try and minimise time spent waiting for the
> > GPU. However, as the igfx and CPU share the same TDP, boosting the CPU
> > frequency will result in the GPU being throttled and its frequency being
> > reduced. Thus declaring iowait negatively impacts on GPU throughput.
> >
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107410
> > References: 52ccc4314293 ("cpufreq: intel_pstate: HWP boost performance on IO wakeup")
> 
> This patch causes up to ~13% performance regressions (with significance
> 5%) on several latency-sensitive tests on my BXT:
> 
>  jxrendermark/rendering-test=Linear Gradient Blend/rendering-size=128x128:     XXX ±35.69% x53 -> XXX ±32.57% x61 d=-13.52% ±31.88% p=2.58%

Curious, as this is just a bunch of composites and as with the others,
should never be latency sensitive (at least under bare X11).
Fwiw, I double checked this result:

Broxton J3455, jxrend -num $(for i in $(seq 1 100); do echo 12 128; done)
x noio-1.txt
+ io-1.txt
+------------------------------------------------------------------------+
|                 +                                                      |
|                 +                                                      |
|                 *                                                      |
|                +*x                                                     |
|              + +***+                                                   |
|              + +***++                                                  |
|              + ****+*   +                                              |
|             ++x******  x+   x                                          |
|       xx    **x*******+x* xx*                                          |
|   + + xx*xx+***********x**x***x x+                                     |
|x x+** x**x****************x***x***+  x + x x                    ++    +|
|           |_______MA_______|                                           |
|          |________MA__________|                                        |
+------------------------------------------------------------------------+
    N           Min           Max        Median           Avg        Stddev
x 100     16109.095     16211.579     16152.497      16154.87     19.270749
+ 100      16116.47     16274.973     16152.365     16156.954     25.304398
No difference proven at 95.0% confidence

Your variance is much, much higher, are you still using the original
jxrendermark that doesn't wait for rendering completion?
-Chris