Quoting Francisco Jerez (2018-07-29 20:29:42) > Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes: > > > Quoting Francisco Jerez (2018-07-28 21:18:50) > >> Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes: > >> > >> > Quoting Francisco Jerez (2018-07-28 06:20:12) > >> >> Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> 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? 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 _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
