Messages in this thread | | | From | Parth Shah <> | Subject | [RFCv3 0/8] TurboSched: A scheduler for sustaining Turbo Frequencies for longer durations | Date | Tue, 25 Jun 2019 10:07:18 +0530 |
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This is the 3rd version of the patchset to sustain Turbo frequencies for longer durations.
The previous versions can be found here: v2: https://lkml.org/lkml/2019/5/15/1258 v1: https://lwn.net/Articles/783959/
The changes in this versions are: v[2] -> v[3]: - Added a new attribute in task_struct to allow per task jitter classification so that scheduler can use this as request to change wakeup path for task packing - Use syscall for jitter classification, removed cgroup based task classification - Use mutex over spinlock to get rid of task sleeping problem - Changed _Bool->int everywhere - Split few patches to have arch specific code separate from core scheduler code ToDo: - Recompute core capacity only during CPU-Hotplug operation - Remove smt capacity
v[1] -> v[2]: - No CPU bound tasks' classification, only jitter tasks are classified from the cpu cgroup controller - Use of Spinlock rather than mutex to count number of jitters in the system classified from cgroup - Architecture specific implementation of Core capacity multiplication factor changes dynamically based on the number of active threads in the core - Selection of non idle core in the system is bounded by DIE domain - Use of UCLAMP mechanism to classify jitter tasks - Removed "highutil_cpu_mask", and rather uses sd for DIE domain to find better fit
Abstract ========
The modern servers allows multiple cores to run at range of frequencies higher than rated range of frequencies. But the power budget of the system inhibits sustaining these higher frequencies for longer durations.
However when certain cores are put to idle states, the power can be effectively channelled to other busy cores, allowing them to sustain the higher frequency.
One way to achieve this is to pack tasks onto fewer cores keeping others idle, but it may lead to performance penalty for such tasks and sustaining higher frequencies proves to be of no benefit. But if one can identify unimportant low utilization tasks which can be packed on the already active cores then waking up of new cores can be avoided. Such tasks are short and/or bursty "jitter tasks" and waking up new core is expensive for such case.
Current CFS algorithm in kernel scheduler is performance oriented and hence tries to assign any idle CPU first for the waking up of new tasks. This policy is perfect for major categories of the workload, but for jitter tasks, one can save energy by packing them onto the active cores and allow those cores to run at higher frequencies.
These patch-set tunes the task wake up logic in scheduler to pack exclusively classified jitter tasks onto busy cores. The work involves the jitter tasks classifications by using syscall based mechanisms.
In brief, if we can pack jitter tasks on busy cores then we can save power by keeping other cores idle and allow busier cores to run at turbo frequencies, patch-set tries to meet this solution in simplest manner. Though, there are some challenges in implementing it(like smt_capacity, un-needed arch hooks, etc) and I'm trying to work around that, it would be great to have a discussion around this patches.
Implementation ==============
These patches uses UCLAMP mechanism[2] used to clamp utilization from the userspace, which can be used to classify the jitter tasks. The task wakeup logic uses this information to pack such tasks onto cores which are already running busy with CPU intensive tasks. The task packing is done at `select_task_rq_fair` only so that in case of wrong decision load balancer may pull the classified jitter tasks for maximizing performance.
Any tasks clamped with cpu.util.max=1 (with sched_setattr syscall) are classified as jitter tasks. We define a core to be non-idle if it is over 12.5% utilized of its capacity; the jitters are packed over these cores using First-fit approach.
To demonstrate/benchmark, one can use a synthetic workload generator `turbo_bench.c`[1] available at https://github.com/parthsl/tools/blob/master/benchmarks/turbo_bench.c
Following snippet demonstrates the use of TurboSched feature: ``` i=8; ./turbo_bench -t 30 -h $i -n $((i*2)) -j ```
Current implementation uses only jitter classified tasks to be packed on the first busy cores, but can be further optimized by getting userspace input of important tasks and keeping track of such tasks. This leads to optimized searching of non idle cores and also more accurate as userspace hints are safer than auto classified busy cores/tasks.
Result ======
The patch-set proves to be useful for the system and the workload where frequency boost is found to be useful than packing tasks into cores. IBM POWER 9 system shows the benefit for a workload can be up to 13%.
Performance benefit of TurboSched w.r.t. CFS +--+--+--+--+--+--+-+--+--+--+--+--+--+--+--+--+--+--+-+--+--+--+--+--+ | + + + + + + + + + + + + + + + + + + + + + + + | 15 +-+ Performance benefit in % +-+ | ** | | ** ** | 10 +-+ ** ** ** +-+ | ** ** ** | | ** ** ** | 5 +-+ ** ** ** ** ** ** +-+ | ** ** ** ** ** ** ** ** | | ** ** ** ** ** ** ** ** ** ** | | * ** ** ** ** ** ** ** ** ** ** ** * | 0 +-+** ** ** ** ** * ** ** ** ** ** ** ** ** ** ** ** * ** ** ** ** **-+ | ** ** ** ** | | ** | -5 +-+ +-+ | + + + + + + + + + + + + + + + + + + + + + + + | +--+--+--+--+--+--+-+--+--+--+--+--+--+--+--+--+--+--+-+--+--+--+--+--+ 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1920 21 22 23 24 No. of workload threads
Frequency benefit of TurboSched w.r.t. CFS +--+--+--+--+--+--+-+--+--+--+--+--+--+--+--+--+--+--+-+--+--+--+--+--+ | + + + + + + + + + + + + + + + + + + + + + + + | 15 +-+ Frequency benefit in % +-+ | ** | | ** | 10 +-+ ** ** +-+ | ** ** ** | | ** ** * ** ** ** | 5 +-+ ** ** ** * ** ** ** ** +-+ | ** ** ** ** * ** ** ** ** ** | | ** ** ** ** ** * ** ** ** ** ** ** | | ** ** ** ** ** * ** ** ** ** ** ** ** ** ** | 0 +-+** ** ** ** ** * ** ** ** ** ** ** ** ** ** ** ** * ** ** ** ** **-+ | | | | -5 +-+ +-+ | + + + + + + + + + + + + + + + + + + + + + + + | +--+--+--+--+--+--+-+--+--+--+--+--+--+--+--+--+--+--+-+--+--+--+--+--+ 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1920 21 22 23 24 No. of workload threads
These numbers are w.r.t. `turbo_bench.c` multi-threaded test benchmark which can create two kinds of tasks: CPU bound (High Utilization) and Jitters (Low Utilization). N in X-axis represents N-CPU bound and N-Jitter tasks spawned.
Series organization ============== - Patches [01-03]: Jitter tasks classification using syscall - Patches [04-05]: Defines Core Capacity to limit task packing - Patches [06-08]: Tune CFS task wakeup logic to pack tasks onto busy cores
Series can be applied on top of Patrick Bellasi's UCLAMP RFCv9[2] patches with branch on tip/sched/core and UCLAMP_TASK_GROUP config options enabled.
References ==========
[1] "Turbo_bench: Synthetic workload generator" https://github.com/parthsl/tools/blob/master/benchmarks/turbo_bench.c
[2] "Patrick Bellasi, Add utilization clamping support" https://lkml.org/lkml/2019/5/15/212
Parth Shah (8): sched/core: Add manual jitter classification using sched_setattr syscall sched: Introduce switch to enable TurboSched mode sched/core: Update turbo_sched count only when required sched/fair: Define core capacity to limit task packing powerpc: Define Core Capacity for POWER systems sched/fair: Tune task wake-up logic to pack jitter tasks sched/fair: Bound non idle core search within LLC domain powerpc: Set turbo domain to NUMA node for task packing
arch/powerpc/include/asm/topology.h | 7 ++ arch/powerpc/kernel/smp.c | 38 ++++++++ include/linux/sched.h | 6 ++ kernel/sched/core.c | 35 +++++++ kernel/sched/fair.c | 141 +++++++++++++++++++++++++++- kernel/sched/sched.h | 9 ++ 6 files changed, 235 insertions(+), 1 deletion(-)
-- 2.17.1
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