sched/doc: Update documentation for base_slice_ns and CONFIG_HZ relation
The tunable base_slice_ns is dependent on CONFIG_HZ (i.e. TICK_NSEC) for any significant performance improvement. The reason being the scheduler tick is not frequent enough to force preemption when base_slice expires in case of: base_slice_ns < TICK_NSEC The below data is of stress-ng: Number of CPU: 1 Stressor threads: 4 Time: 30sec On CONFIG_HZ=1000 | base_slice | avg-run (msec) | context-switches | | ---------- | -------------- | ---------------- | | 3ms | 2.914 | 10342 | | 6ms | 4.857 | 6196 | | 9ms | 6.754 | 4482 | | 12ms | 7.872 | 3802 | | 22ms | 11.294 | 2710 | | 32ms | 13.425 | 2284 | On CONFIG_HZ=100 | base_slice | avg-run (msec) | context-switches | | ---------- | -------------- | ---------------- | | 3ms | 9.144 | 3337 | | 6ms | 9.113 | 3301 | | 9ms | 8.991 | 3315 | | 12ms | 12.935 | 2328 | | 22ms | 16.031 | 1915 | | 32ms | 18.608 | 1622 | base_slice: the value of base_slice in ms avg-run (msec): average time of the stressor threads got on cpu before it got preempted context-switches: number of context switches for the stress-ng process Signed-off-by: Mukesh Kumar Chaurasiya <mchauras@linux.ibm.com> Signed-off-by: Ingo Molnar <mingo@kernel.org> Cc: Randy Dunlap <rdunlap@infradead.org> Link: https://lore.kernel.org/r/20240320173815.927637-2-mchauras@linux.ibm.com
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@ -100,6 +100,9 @@ which can be used to tune the scheduler from "desktop" (i.e., low latencies) to
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"server" (i.e., good batching) workloads. It defaults to a setting suitable
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"server" (i.e., good batching) workloads. It defaults to a setting suitable
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for desktop workloads. SCHED_BATCH is handled by the CFS scheduler module too.
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for desktop workloads. SCHED_BATCH is handled by the CFS scheduler module too.
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In case CONFIG_HZ results in base_slice_ns < TICK_NSEC, the value of
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base_slice_ns will have little to no impact on the workloads.
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Due to its design, the CFS scheduler is not prone to any of the "attacks" that
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Due to its design, the CFS scheduler is not prone to any of the "attacks" that
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exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,
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exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,
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chew.c, ring-test.c, massive_intr.c all work fine and do not impact
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chew.c, ring-test.c, massive_intr.c all work fine and do not impact
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