Hi Steve, On Mon, 22 Oct 2018 at 17:10, Steve Sistare <steven.sist...@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 > > 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. >
