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Message-ID: <20140514125515.GK13658@twins.programming.kicks-ass.net>
Date: Wed, 14 May 2014 14:55:15 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "xiaofeng.yan" <xiaofeng.yan@...wei.com>
Cc: Ingo Molnar <mingo@...hat.com>, duzhiping.du@...wei.com,
xiaofeng.yan2012@...il.com, juri.lelli@...il.com,
luca.abeni@...tn.it, raistlin@...ux.it, henrik@...tad.us,
tkhai@...dex.ru, harald.gustafsson@...csson.com,
linux-kernel@...r.kernel.org
Subject: Re: [RFD] sched/deadline: EDF dynamic quota design
On Wed, May 14, 2014 at 01:32:45PM +0200, Peter Zijlstra wrote:
> On Wed, May 14, 2014 at 06:43:38PM +0800, xiaofeng.yan wrote:
> > Hi all,
> >
> > We design dynamic quota function based on current EDF schedule .
> > Wehave realized this dynamic quota design and get good performance in the actual scene.
> > The impalement for this function is very strong connected with production line requirement.
> > In some implementations, the current design in product could be be accepted by community,
> > such as beyond the total bandwidth limitations.
> > So we design some parts again.
> > If you want to review our current implements in product, I will send our patches and test result.
> > We hope to push it to the main line. Could you give me suggestion whether it can be accepted or not?
> > We will be very grateful for your suggestion.
>
> At the very least you could've Cc'ed the people who actually wrote the
> EDF stuff :-/
Also, Cc lkml, left the rest of the msg intact as clearly people haven't
had a copy yet.
> > The design principle is as follows:
> >
> >
> > ------------------EDF Dynamic Quota Design------------------
> >
> > * Current EDF defect
> > EDF tasks' bandwidth is fixed currently. they could not adjust
> > their quota whenever they are busy or idle.
> >
> > * The scene for dynamic quota
> > Currently, we have the scenarios which need to adjust tasks'
> > quota dynamically. Such as, the network's workload fluctuates when
> > forwarding the packets, which results in imbalance. Busy task should
> > increase quota, and idle task should reduce quota.
> > It is beneficial to increase the processing ability of the business.
> >
> > * Dynamic quota idea
> > We add the dynamic quota function based on current EDF schedule.
> > The principle is as follows:
> > The total bandwidth of EDF tasks is fixed during running.
> > Idle tasks release left bandwidth to the global bandwidth pool, and
> > busy tasks get some quota from the global pool.
> >
> > * Example.
> > Three tasks: T1,T2,T3. Their initial status is as follows,
> > T1(200us,500us,500us)
> > T2(200us,500us,500us)
> > T3(200us,500us,500us)
> >
> > At time t1, the tasks' running status is as follows,
> > T1(200us,500us,500us)
> > T2(100us,500us,500us)
> > T3(200us,500us,500us)
> > Busy tasks: T1,T3
> > Idle task: T2
> > Bandwidth pool: 20%
> >
> > Now, there are 20% quota in the bandwidth pool, T1 or T3 get 5% quota
> > (adjustable) from the bandwidth pool when they enter run-queue.
> > Then, the status is as follows:
> > T1(225us,500us,500us)
> > T2(100us,500us,500us)
> > T3(225us,500us,500us)
> > Bandwidth pool: 10%
> >
> > Busy tasks could get the quota when the bandwidth pool is not empty.
> >
> > ----------------------------------------------------------------------------------------
>
> Yeah, not sure that's sound. But I'll leave that to others.
>
> So there have been papers on how if you transform the runtime into an
> avg the tardiness also turns into an avg and remains bounded.
>
> Now, you still have to guarantee your individual tasks respect the avg,
> otherwise the tardiness guarantees are out the window along with it.
>
> I've not thought through your proposal to see if it maintains this
> guarantee -- but seeing how you don't talk about guarantees at all I
> fear the worst.
>
> Another point is that 'avg' or 'dynamic' tasks should co-exist with the
> normal fixed runtime tasks (ideally) without affecting the performance
> of the normal tasks (too much).
>
> Your proposal also doesn't cover this.
>
> There have also been proposals to turn the CBS into a 'soft' CBS and
> instead of hard throttling allow the task to continue executing at a
> lower class (throttle in effect turns into a switch to SCHED_NORMAL, and
> unthrottle restores it to SCHED_DEADLINE).
>
>
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