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SubjectRe: [PATCH 00/10] steal tasks to improve CPU utilization
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On 10/25/2018 8:43 AM, Vincent Guittot wrote:
> On Thu, 25 Oct 2018 at 13:29, Steven Sistare <steven.sistare@oracle.com> wrote:
>>
>> 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.
>
> ok. I haven't noticed that later explanation.
>
> You said " Note: for all hackbench runs, sched_wakeup_granularity_ns
> is set to 15 msec. Otherwise, preemptions increase at higher loads and
> distort the comparison between baseline and new."
>
> What do you mean exactly by distort ?

With the default value of sched_wakeup_granularity_ns and the load range
I tested, as load and CPU utilization increases, preemptions increase, average
timeslice decreases, and time per task goes up. For a given task count, stealing
achieves higher utilization than base for the same count, so is hit harder by the
preemption effect than the base. Raising sched_wakeup_granularity_ns factors this
out of the comparison.

- 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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