linux-kernel - Re: [PATCH] sched/doc: Update documentation for base_slice_ns and CONFIG

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Message-ID: <51d825a3-46a7-42eb-a413-4beb37b7656d@infradead.org>
Date: Thu, 21 Mar 2024 07:55:56 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Mukesh Kumar Chaurasiya <mchauras@...ux.ibm.com>,
 linux-doc@...r.kernel.org
Cc: corbet@....net, mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/doc: Update documentation for base_slice_ns and
 CONFIG_HZ relation

Hi,

On 3/20/24 10:38, Mukesh Kumar Chaurasiya wrote:
> 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@...ux.ibm.com>
> ---
>  Documentation/scheduler/sched-design-CFS.rst | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/Documentation/scheduler/sched-design-CFS.rst b/Documentation/scheduler/sched-design-CFS.rst
> index 6cffffe26500..82985d675554 100644
> --- a/Documentation/scheduler/sched-design-CFS.rst
> +++ b/Documentation/scheduler/sched-design-CFS.rst
> @@ -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 the CONFIG_HZ leads to base_slice_ns < TICK_NSEC. The settings of

                                                  TICK_NSEC, the {setting, value}

> +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

-- 
#Randy