linux-kernel - Re: [RFC PATCH 1/9] sched,cgroup: Add interface for latency-nice

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Message-ID: <20190905114725.ehi5ea6qg3rychlz@e107158-lin.cambridge.arm.com>
Date:   Thu, 5 Sep 2019 12:47:26 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        Subhra Mazumdar <subhra.mazumdar@...cle.com>,
        linux-kernel@...r.kernel.org, mingo@...hat.com, tglx@...utronix.de,
        steven.sistare@...cle.com, dhaval.giani@...cle.com,
        daniel.lezcano@...aro.org, vincent.guittot@...aro.org,
        viresh.kumar@...aro.org, tim.c.chen@...ux.intel.com,
        mgorman@...hsingularity.net, parth@...ux.ibm.com
Subject: Re: [RFC PATCH 1/9] sched,cgroup: Add interface for latency-nice

On 09/05/19 13:30, Peter Zijlstra wrote:
> On Thu, Sep 05, 2019 at 12:13:47PM +0100, Qais Yousef wrote:
> > On 09/05/19 12:46, Peter Zijlstra wrote:
> 
> > > 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.
> > 
> > Another use case I'm considering is using latency-nice to prefer an idle CPU if
> > latency-nice is set otherwise go for the most energy efficient CPU.
> > 
> > Ie: sacrifice (some) energy for latency.
> > 
> > The way I see interpreting latency-nice here as a binary switch. But
> > maybe we can use the range to select what (some) energy to sacrifice
> > mean here. Hmmm.
> 
> It cannot be binary, per definition is must be ternary, that is, <0, ==0
> and >0 (or middle value if you're of that persuasion).

I meant I want to use it as a binary.

> 
> In your case, I'm thinking you mean >0, we want to lower the latency.

Yes. As long as there's an easy way to say: does this task care about latency
or not I'm good.

> 
> Anyway; there were a number of things mentioned at OSPM that we could
> tie into this thing and finding sensible mappings is going to be a bit
> of trial and error I suppose.
> 
> But as patrick said; we're very much exporting a BIAS knob, not a set of
> behaviours.

Agreed. I just wanted to say that the way this range is going to be
interpreted will differ from path to path and we need to consider that in the
final mapping. Especially from the final user's perspective of what setting
this value ultimately means to them.

--
Qais Yousef