Messages in this thread | | | From | Vincent Guittot <> | Date | Mon, 10 Dec 2018 18:08:22 +0100 | Subject | Re: [PATCH v4 00/10] steal tasks to improve CPU utilization |
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On Mon, 10 Dec 2018 at 17:33, Vincent Guittot <vincent.guittot@linaro.org> wrote: > > On Mon, 10 Dec 2018 at 17:29, Steven Sistare <steven.sistare@oracle.com> wrote: > > > > On 12/10/2018 11:10 AM, Vincent Guittot wrote: > > > Hi Steven, > > > > > > On Thu, 6 Dec 2018 at 22:38, Steve Sistare <steven.sistare@oracle.com> wrote: > > >> > > >> When a CPU has no more CFS tasks to run, and idle_balance() fails to > > >> find a task, then attempt to steal a task from an overloaded CPU in the > > >> same LLC. Maintain and use a bitmap of overloaded CPUs to efficiently > > >> identify candidates. To minimize search time, steal the first migratable > > >> task that is found when the bitmap is traversed. For fairness, search > > >> for migratable tasks on an overloaded CPU in order of next to run. > > >> > > >> This simple stealing yields a higher CPU utilization than idle_balance() > > >> alone, because the search is cheap, so it may be called every time the CPU > > >> is about to go idle. idle_balance() does more work because it searches > > >> widely for the busiest queue, so to limit its CPU consumption, it declines > > >> to search if the system is too busy. Simple stealing does not offload the > > >> globally busiest queue, but it is much better than running nothing at all. > > >> > > >> The bitmap of overloaded CPUs is a new type of sparse bitmap, designed to > > >> reduce cache contention vs the usual bitmap when many threads concurrently > > >> set, clear, and visit elements. > > >> > > >> Patch 1 defines the sparsemask type and its operations. > > >> > > >> Patches 2, 3, and 4 implement the bitmap of overloaded CPUs. > > >> > > >> Patches 5 and 6 refactor existing code for a cleaner merge of later > > >> patches. > > >> > > >> Patches 7 and 8 implement task stealing using the overloaded CPUs bitmap. > > >> > > >> Patch 9 disables stealing on systems with more than 2 NUMA nodes for the > > >> time being because of performance regressions that are not due to stealing > > >> per-se. See the patch description for details. > > >> > > >> Patch 10 adds schedstats for comparing the new behavior to the old, and > > >> provided as a convenience for developers only, not for integration. > > >> > > >> The patch series is based on kernel 4.20.0-rc1. It compiles, boots, and > > >> runs with/without each of CONFIG_SCHED_SMT, CONFIG_SMP, CONFIG_SCHED_DEBUG, > > >> and CONFIG_PREEMPT. It runs without error with CONFIG_DEBUG_PREEMPT + > > >> CONFIG_SLUB_DEBUG + CONFIG_DEBUG_PAGEALLOC + CONFIG_DEBUG_MUTEXES + > > >> CONFIG_DEBUG_SPINLOCK + CONFIG_DEBUG_ATOMIC_SLEEP. CPU hot plug and CPU > > >> bandwidth control were tested. > > >> > > >> Stealing improves utilization with only a modest CPU overhead in scheduler > > >> code. In the following experiment, hackbench is run with varying numbers > > >> of groups (40 tasks per group), and the delta in /proc/schedstat is shown > > >> for each run, averaged per CPU, augmented with these non-standard stats: > > >> > > >> %find - percent of time spent in old and new functions that search for > > >> idle CPUs and tasks to steal and set the overloaded CPUs bitmap. > > >> > > >> steal - number of times a task is stolen from another CPU. > > >> > > >> X6-2: 1 socket * 10 cores * 2 hyperthreads = 20 CPUs > > >> Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz > > >> hackbench <grps> process 100000 > > >> sched_wakeup_granularity_ns=15000000 > > >> > > >> baseline > > >> grps time %busy slice sched idle wake %find steal > > >> 1 8.084 75.02 0.10 105476 46291 59183 0.31 0 > > >> 2 13.892 85.33 0.10 190225 70958 119264 0.45 0 > > >> 3 19.668 89.04 0.10 263896 87047 176850 0.49 0 > > >> 4 25.279 91.28 0.10 322171 94691 227474 0.51 0 > > >> 8 47.832 94.86 0.09 630636 144141 486322 0.56 0 > > >> > > >> new > > >> grps time %busy slice sched idle wake %find steal %speedup > > >> 1 5.938 96.80 0.24 31255 7190 24061 0.63 7433 36.1 > > >> 2 11.491 99.23 0.16 74097 4578 69512 0.84 19463 20.9 > > >> 3 16.987 99.66 0.15 115824 1985 113826 0.77 24707 15.8 > > >> 4 22.504 99.80 0.14 167188 2385 164786 0.75 29353 12.3 > > >> 8 44.441 99.86 0.11 389153 1616 387401 0.67 38190 7.6 > > >> > > >> Elapsed time improves by 8 to 36%, and CPU busy utilization is up > > >> by 5 to 22% hitting 99% for 2 or more groups (80 or more tasks). > > >> The cost is at most 0.4% more find time. > > > > > > I have run some hackbench tests on my hikey arm64 octo cores with your > > > patchset. My original intent was to send a tested-by but I have some > > > performances regressions. > > > This hikey is the smp one and not the asymetric hikey960 that Valentin > > > used for his tests > > > The sched domain topology is > > > domain-0: span=0-3 level=MC and domain-0: span=4-7 level=MC > > > domain-1: span=0-7 level=DIE > > > > > > I have run 12 times hackbench -g $j -P -l 2000 with j equals to 1 2 3 4 8 > > > > > > grps time > > > 1 1.396 > > > 2 2.699 > > > 3 3.617 > > > 4 4.498 > > > 8 7.721 > > > > > > Then after disabling STEAL in sched_feature with echo NO_STEAL > > > > /sys/kernel/debug/sched_features , the results become: > > > grps time > > > 1 1.217 > > > 2 1.973 > > > 3 2.855 > > > 4 3.932 > > > 8 7.674 > > > > > > I haven't looked in details about some possible reasons of such > > > difference yet and haven't collected the stats that you added with > > > patch 10. > > > Have you got a script to collect and post process them ? > > > > > > Regards, > > > Vincent > > > > Thanks Vincent. What is the value of /proc/sys/kernel/sched_wakeup_granularity_ns? > > it's 4000000 > > > Try 15000000. Your 8-core system is heavily overloaded with 40 * groups tasks, > > and I suspect preemptions are killing performance. > > ok. I'm going to run the tests with the proposed value
Results look better after changing /proc/sys/kernel/sched_wakeup_granularity_ns
With STEAL grps time 1 0.869 2 1.646 3 2.395 4 3.163 8 6.199
after echo NO_STEAL > /sys/kernel/debug/sched_features grps time 1 0.928 2 1.770 3 2.597 4 3.407 8 6.431
There is a 7% improvement with steal and the larger value for /proc/sys/kernel/sched_wakeup_granularity_ns for all groups Should we set the STEAL feature disabled by default as this provides benefit only when changing sched_wakeup_granularity_ns value from default value?
> > > > > I have a python script to post-process schedstat files, but it does many things > > and is large and I am not ready to share it. I can write a short bash script if > > that would help. > > It was mainly in case you wanted the figures of these statistics > > Vincent > > > > > - Steve
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