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

[Francisco Jerez]

Chris Wilson <chris at chris-wilson.co.uk> writes:

> Quoting Francisco Jerez (2018-07-29 20:29:42)
>> Chris Wilson <chris at chris-wilson.co.uk> writes:
>> 
>> > Quoting Francisco Jerez (2018-07-28 21:18:50)
>> >> Chris Wilson <chris at chris-wilson.co.uk> writes:
>> >> 
>> >> > 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%
>> >> >
>> >> 
>> >> The jxrendermark Linear Gradient Blend test-case had probably the
>> >> smallest effect size of all the regressions I noticed...  Can you take a
>> >> look at any of the other ones instead?
>> >
>> > It was the biggest in the list, was it not? I didn't observe anything of
>> > note in a quick look at x11perf, but didn't let it run for a good sample
>> > size. They didn't seem to be as relevant as jxrendermark so I went and
>> > dug that out.
>> >
>> 
>> That was the biggest regression in absolute value, but the smallest in
>> effect size (roughly 0.4 standard deviations).
>
> d=-13.52% wasn't the delta between the two runs?
>

It is, less than half of 31.88% which is the pooled standard deviation.

> Sorry, but it appears to be redacted beyond my comprehension.
>
>> >> > Curious, as this is just a bunch of composites and as with the others,
>> >> > should never be latency sensitive (at least under bare X11).
>> >> 
>> >> They are largely latency-sensitive due to the poor pipelining they seem
>> >> to achieve between their GPU rendering work and the X11 thread.
>> >
>> > Only the X11 thread is touching the GPU, and in the cases I looked at
>> > it, we were either waiting for the ring to drain or on throttling.
>> > Synchronisation with the GPU was only for draining the queue on timing,
>> > and the cpu was able to stay ahead during the benchmark.
>> >
>> 
>> Apparently the CPU doesn't get ahead enough for the GPU to be
>> consistently loaded, which prevents us from hiding the latency of the
>> CPU computation even in those cases.
>
> The curse of reproducibility. On my bxt, I don't see the issue, so we
> have a significant difference in setup.
> -Chris
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