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From: Peter Zijlstra <peterz@infradead.org>
To: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Subhra Mazumdar <subhra.mazumdar@oracle.com>,
	linux-kernel@vger.kernel.org, mingo@redhat.com,
	tglx@linutronix.de, steven.sistare@oracle.com,
	dhaval.giani@oracle.com, daniel.lezcano@linaro.org,
	vincent.guittot@linaro.org, viresh.kumar@linaro.org,
	tim.c.chen@linux.intel.com, mgorman@techsingularity.net,
	parth@linux.ibm.com
Subject: Re: [RFC PATCH 1/9] sched,cgroup: Add interface for latency-nice
Date: Thu, 5 Sep 2019 12:46:16 +0200
Message-ID: <20190905104616.GD2332@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <87r24v2i14.fsf@arm.com>

On Thu, Sep 05, 2019 at 10:45:27AM +0100, Patrick Bellasi wrote:

> > From just reading the above, I would expect it to have the range
> > [-20,19] just like normal nice. Apparently this is not so.
> 
> Regarding the range for the latency-nice values, I guess we have two
> options:
> 
>   - [-20..19], which makes it similar to priorities
>   downside: we quite likely end up with a kernel space representation
>   which does not match the user-space one, e.g. look at
>   task_struct::prio.
> 
>   - [0..1024], which makes it more similar to a "percentage"
> 
> Being latency-nice a new concept, we are not constrained by POSIX and
> IMHO the [0..1024] scale is a better fit.
> 
> That will translate into:
> 
>   latency-nice=0 : default (current mainline) behaviour, all "biasing"
>   policies are disabled and we wakeup up as fast as possible
> 
>   latency-nice=1024 : maximum niceness, where for example we can imaging
>   to turn switch a CFS task to be SCHED_IDLE?

There's a few things wrong there; I really feel that if we call it nice,
it should be like nice. Otherwise we should call it latency-bias and not
have the association with nice to confuse people.

Secondly; the default should be in the middle of the range. Naturally
this would be a signed range like nice [-(x+1),x] for some x. but if you
want [0,1024], then the default really should be 512, but personally I
like 0 better as a default, in which case we need negative numbers.

This is important because we want to be able to bias towards less
importance to (tail) latency as well as more importantance to (tail)
latency.

Specifically, Oracle wants to sacrifice (some) latency for throughput.
Facebook OTOH seems to want to sacrifice (some) throughput for latency.

  reply index

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-30 17:49 [RFC PATCH 0/9] Task latency-nice subhra mazumdar
2019-08-30 17:49 ` [RFC PATCH 1/9] sched,cgroup: Add interface for latency-nice subhra mazumdar
2019-09-04 17:32   ` Tim Chen
2019-09-05  6:15     ` Parth Shah
2019-09-05 10:11       ` Patrick Bellasi
2019-09-06 12:22         ` Parth Shah
2019-09-05  8:31   ` Peter Zijlstra
2019-09-05  9:45     ` Patrick Bellasi
2019-09-05 10:46       ` Peter Zijlstra [this message]
2019-09-05 11:13         ` Qais Yousef
2019-09-05 11:30           ` Peter Zijlstra
2019-09-05 11:40             ` Patrick Bellasi
2019-09-05 11:48               ` Peter Zijlstra
2019-09-05 13:32                 ` Qais Yousef
2019-09-05 11:47             ` Qais Yousef
2020-04-16  0:02               ` Joel Fernandes
2020-04-16 17:23                 ` Dietmar Eggemann
2020-04-18 16:01                   ` Joel Fernandes
2020-04-20 11:26                     ` Parth Shah
2020-04-20 19:14                       ` Joel Fernandes
2020-04-20 11:47                     ` Qais Yousef
2020-04-20 19:10                       ` Joel Fernandes
2019-09-05 11:30           ` Patrick Bellasi
2019-09-05 11:47             ` Peter Zijlstra
2019-09-05 11:18         ` Patrick Bellasi
2019-09-05 11:40           ` Peter Zijlstra
2019-09-05 11:46             ` Patrick Bellasi
2019-09-05 11:46           ` Valentin Schneider
2019-09-05 13:07             ` Patrick Bellasi
2019-09-05 14:48               ` Valentin Schneider
2019-09-06 12:45               ` Parth Shah
2019-09-06 14:13                 ` Valentin Schneider
2019-09-06 14:32                   ` Vincent Guittot
2019-09-06 17:10                   ` Parth Shah
2019-09-06 22:50                     ` Valentin Schneider
2019-09-06 12:31       ` Parth Shah
2019-09-05 10:05   ` Patrick Bellasi
2019-09-05 10:48     ` Peter Zijlstra
2019-08-30 17:49 ` [RFC PATCH 2/9] sched: add search limit as per latency-nice subhra mazumdar
2019-09-05  6:22   ` Parth Shah
2019-08-30 17:49 ` [RFC PATCH 3/9] sched: add sched feature to disable idle core search subhra mazumdar
2019-09-05 10:17   ` Patrick Bellasi
2019-09-05 22:02     ` Subhra Mazumdar
2019-08-30 17:49 ` [RFC PATCH 4/9] sched: SIS_CORE " subhra mazumdar
2019-09-05 10:19   ` Patrick Bellasi
2019-08-30 17:49 ` [RFC PATCH 5/9] sched: Define macro for number of CPUs in core subhra mazumdar
2019-08-30 17:49 ` [RFC PATCH 6/9] x86/smpboot: Optimize cpumask_weight_sibling macro for x86 subhra mazumdar
2019-08-30 17:49 ` [RFC PATCH 7/9] sched: search SMT before LLC domain subhra mazumdar
2019-09-05  9:31   ` Peter Zijlstra
2019-09-05 20:40     ` Subhra Mazumdar
2019-08-30 17:49 ` [RFC PATCH 8/9] sched: introduce per-cpu var next_cpu to track search limit subhra mazumdar
2019-08-30 17:49 ` [RFC PATCH 9/9] sched: rotate the cpu search window for better spread subhra mazumdar
2019-09-05  6:37   ` Parth Shah
2019-09-05  5:55 ` [RFC PATCH 0/9] Task latency-nice Parth Shah
2019-09-05 10:31 ` Patrick Bellasi

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