Messages in this thread | | | Subject | Re: [PATCH v4 00/10] steal tasks to improve CPU utilization | From | Steven Sistare <> | Date | Mon, 10 Dec 2018 11:29:08 -0500 |
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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? Try 15000000. Your 8-core system is heavily overloaded with 40 * groups tasks, and I suspect preemptions are killing performance.
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.
- Steve
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