linux-kernel - Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new forked task

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Message-ID: <CAPM31RK06z9SrwACTCyOg96tza9UxA9ZRZegBDA76pG2oM+m7A@mail.gmail.com>
Date:	Wed, 8 May 2013 05:00:34 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Alex Shi <alex.shi@...el.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>
Subject: Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new
 forked task

On Wed, May 8, 2013 at 4:34 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, May 07, 2013 at 04:20:55AM -0700, Paul Turner wrote:
>> Yes, 1024 was only intended as a starting point.  We could also
>> arbitrarily pick something larger, the key is that we pick
>> _something_.
>>
>> If we wanted to be more exacting about it we could just give them a
>> sched_slice() worth; this would have a few obvious "nice" properties
>> (pun intended).
>
> Oh I see I misunderstood again :/ Its not about the effective load but weight
> of the initial effective load wrt adjustment.
>
> Previous schedulers didn't have this aspect at all, so no experience from me
> here. Paul would be the one, since he's ran longest with this stuff.
>
> That said, I would tend to keep it shorter rather than longer so that it would
> adjust quicker to whatever it really wanted to be.
>
> Morten says the load is unstable specifically on loaded systems.

Here, Morten was (I believe) referring to the stability at task startup.

To be clear:
Because we have such a small runnable period denominator at this point
a single changed observation (for an equivalently behaving thread)
could have a very large effect.  e.g. fork/exec -- happen to take a
major #pf, observe a "relatively" long initial block.

By associating an initial period (along with our full load_contrib)
here, we're making the denominator larger so that these effects are
less pronounced; achieving better convergence towards what our load
contribution should actually be.

Also: We do this conservatively, by converging down, not up.

> I would think
> this is because we'd experience scheduling latency, we're runnable more pushing
> things up. But if we're really an idle task at heart we'd not run again for a
> long while, pushing things down again.

Exactly, this is why we must be careful to use instaneous weights
about wake-up decisions.  Interactive and background tasks are largely
idle.

While this is exactly how we want them to be perceived from a
load-balance perspective it's important to keep in mind that while
wake-up placement has a very important role in the overall balance of
a system, it is not playing quite the same game as the load-balancer.

>
> So on that point Paul's suggestion of maybe starting with __sched_slice() might
> make sense because it increases the weight of the initial avg with nr_running.
> Not sure really, we'll have to play and see what works best for a number of
> workloads.
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