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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
This commit is contained in:
Mukesh Kumar Chaurasiya 2024-03-20 23:08:16 +05:30 committed by Ingo Molnar
parent 4ae3dc83b0
commit a26979377b
1 changed files with 3 additions and 0 deletions
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3
Documentation/scheduler/sched-design-CFS.rst
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@ -100,6 +100,9 @@ which can be used to tune the scheduler from "desktop" (i.e., low latencies) to
"server" (i.e., good batching) workloads. It defaults to a setting suitable
for desktop workloads. SCHED_BATCH is handled by the CFS scheduler module too.
In case CONFIG_HZ results in base_slice_ns < TICK_NSEC, the value of
base_slice_ns will have little to no impact on the workloads.
Due to its design, the CFS scheduler is not prone to any of the "attacks" that
exist today against the heuristics of the stock scheduler: fiftyp.c, thud.c,
chew.c, ring-test.c, massive_intr.c all work fine and do not impact