Reduce migrations and unnecessary spreading of load to multiple CPUs
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- v1 Latest4 patches 39 repliesIn review
Patches in v1 No applies recorded yet
| # | Subject and files | Size | Replies | Applied |
|---|---|---|---|---|
| 1/4 | sched/fair: Remove unnecessary parameters from wake_affine_idlekernel/ | +3 | 0 replies | |
| 2/4 | sched/fair: Restructure wake_affine to return a CPU idkernel/ | +17 | 0 replies | |
| 3/4 | sched/fair: Do not migrate if the prev_cpu is idlekernel/ | +7 | 0 replies | |
| 4/4 | sched/fair: Use a recently used CPU as an idle candidate and the basis for SISinclude/ kernel/ kernel/ | +29 | 39 replies |
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Cover letter [PATCH 0/4] Reduce migrations and unnecessary spreading of load to multiple CPUs
It has been observed recently that problems with interaction between scheduler decisions and cpufreq decisions have been unfavourable. The scheduler decisions are biased towards reducing latency of searches which is great on one hand but tends to spread load across an entire socket unnecessarily. On low utilisation, this means the load on each individual CPU is low which can be good but cpufreq decides that utilisation on individual CPUs is too low to increase P-state and overall throughput suffers.
When a cpufreq driver is completely under the control of the OS, it can be compensated for. For example, intel_pstate can decide to boost apparent utilisation if a task recently slept on a CPU for idle. However, if hardware-based cpufreq is in play (e.g. HWP) then very poor decisions can be made and the OS cannot do much about it. This only gets worse as HWP becomes more prevalent, sockets get larger and the p-state for individual cores can be controlled. Just setting the performance governor is not an answer given that plenty of people really do worry about power utilisation and still want a reasonable balance between performance and power.
Patches 0-3 of this series reduce the number of migrations due to interrupts.
Specifically, if prev CPU and the current CPU share cache and prev CPU is idle then use it in preference to migrating the load. The full reasoning why is in the changelog.
Patch 4 observes that co-operating tasks, particularly between an application
and a kworker can push a load around a socket very quickly. This is particularly true if interrupts are involved (e.g. IO completions) and are delivered round-robin (e.g. due to MSI-X). It tracks what CPU was used for a recent wakeup and reuses that CPU if it's still idle when woken later. This reduces the number of cores that are active for a workload and can have a big boost in throughput without a hit to wakeup latency or stacking multiple tasks on one CPU when a machine is lightly loaded.
include/linux/sched.h | 8 ++++++ kernel/sched/core.c | 1 + kernel/sched/fair.c | 69 +++++++++++++++++++++++++++++++++------------------ 3 files changed, 54 insertions(+), 24 deletions(-)
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2.15.1