Messages in this thread | | | Subject | Re: [PATCH 00/10] steal tasks to improve CPU utilization | From | Steven Sistare <> | Date | Thu, 25 Oct 2018 07:28:49 -0400 |
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On 10/25/2018 3:50 AM, Vincent Guittot wrote: > Hi Steve, > > On Mon, 22 Oct 2018 at 17:10, 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 4.19.0-rc7. 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 imprroves 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 > > Why do you mention this sched_wakeup_granularity_ns value ? > It is something that you changed for you tests ? > The comment for this tunable says that default value is 1ms * > ilog(ncpus) = 4ms for 20CPUs
I changed it for the test, and I explain why a few paragraphs later. The value matches the one set by tuned.service, for those running it.
- Steve
> >> >> 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. > >>
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