Messages in this thread | | | Date | Tue, 22 Jun 2021 15:19:34 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v6 1/3] sched/fair: Introduce the burstable CFS controller |
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On Mon, Jun 21, 2021 at 05:27:58PM +0800, Huaixin Chang wrote: > The CFS bandwidth controller limits CPU requests of a task group to > quota during each period. However, parallel workloads might be bursty > so that they get throttled even when their average utilization is under > quota. And they are latency sensitive at the same time so that > throttling them is undesired. > > We borrow time now against our future underrun, at the cost of increased > interference against the other system users. All nicely bounded. > > Traditional (UP-EDF) bandwidth control is something like: > > (U = \Sum u_i) <= 1 > > This guaranteeds both that every deadline is met and that the system is > stable. After all, if U were > 1, then for every second of walltime, > we'd have to run more than a second of program time, and obviously miss > our deadline, but the next deadline will be further out still, there is > never time to catch up, unbounded fail. > > This work observes that a workload doesn't always executes the full > quota; this enables one to describe u_i as a statistical distribution. > > For example, have u_i = {x,e}_i, where x is the p(95) and x+e p(100) > (the traditional WCET). This effectively allows u to be smaller, > increasing the efficiency (we can pack more tasks in the system), but at > the cost of missing deadlines when all the odds line up. However, it > does maintain stability, since every overrun must be paired with an > underrun as long as our x is above the average. > > That is, suppose we have 2 tasks, both specify a p(95) value, then we > have a p(95)*p(95) = 90.25% chance both tasks are within their quota and > everything is good. At the same time we have a p(5)p(5) = 0.25% chance > both tasks will exceed their quota at the same time (guaranteed deadline > fail). Somewhere in between there's a threshold where one exceeds and > the other doesn't underrun enough to compensate; this depends on the > specific CDFs. > > At the same time, we can say that the worst case deadline miss, will be > \Sum e_i; that is, there is a bounded tardiness (under the assumption > that x+e is indeed WCET). > > The benefit of burst is seen when testing with schbench. Default value of > kernel.sched_cfs_bandwidth_slice_us(5ms) and CONFIG_HZ(1000) is used. > > mkdir /sys/fs/cgroup/cpu/test > echo $$ > /sys/fs/cgroup/cpu/test/cgroup.procs > echo 100000 > /sys/fs/cgroup/cpu/test/cpu.cfs_quota_us > echo 100000 > /sys/fs/cgroup/cpu/test/cpu.cfs_burst_us > > ./schbench -m 1 -t 3 -r 20 -c 80000 -R 10 > > The average CPU usage is at 80%. I run this for 10 times, and got long tail > latency for 6 times and got throttled for 8 times. > > Tail latencies are shown below, and it wasn't the worst case. > > Latency percentiles (usec) > 50.0000th: 19872 > 75.0000th: 21344 > 90.0000th: 22176 > 95.0000th: 22496 > *99.0000th: 22752 > 99.5000th: 22752 > 99.9000th: 22752 > min=0, max=22727 > rps: 9.90 p95 (usec) 22496 p99 (usec) 22752 p95/cputime 28.12% p99/cputime 28.44% > > The interferenece when using burst is valued by the possibilities for > missing the deadline and the average WCET. Test results showed that when > there many cgroups or CPU is under utilized, the interference is > limited. More details are shown in: > https://lore.kernel.org/lkml/5371BD36-55AE-4F71-B9D7-B86DC32E3D2B@linux.alibaba.com/ > > Co-developed-by: Shanpei Chen <shanpeic@linux.alibaba.com> > Signed-off-by: Shanpei Chen <shanpeic@linux.alibaba.com> > Co-developed-by: Tianchen Ding <dtcccc@linux.alibaba.com> > Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com> > Signed-off-by: Huaixin Chang <changhuaixin@linux.alibaba.com> > ---
Ben, what say you? I'm tempted to pick up at least this first patch.
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