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). >> > 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. > Off the top of my head, for X to be latency sensitive you need to mix > client and Xserver rendering, along the lines of Paint; GetImage, in the > extreme becoming gem_sync. Adding a compositor is also interesting for > the context switching will prevent us merging requests (but that all > depends on the frequency of compositor updates ofc), and we would > need more CPU and require reasonably low latency (less than the next > request) to keep the GPU busy. However, that is driven directly off > interrupts, iowait isn't a factor -- but your hook could still be useful > to provide pm_qos. > -Chris
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