Re: [ANNOUNCE][RFC] PlugSched-6.3.1 for 2.6.16-rc5

[Peter Williams <pwil3058@bigpond.net.au>]

Al Boldi wrote:
> Peter Williams wrote:
>> Al Boldi wrote:
>>> But how does this explain spa_no_frills setting promotion to max not
>>> having this problem?
>> I'm still puzzled by this.  The only thing I can think of is that the
>> promotion mechanism is to simple in that it just moves all promotable
>> tasks up one slot without regard for how long they've been on the queue.
>>   Doing this was a deliberate decision based on the desire to minimize
>> overhead and the belief that it wouldn't matter in the grand scheme of
>> things.  I may do some experimenting with slightly more sophisticated
>> version.
>>
>> Properly done, promotion should hardly ever occur but the cost would be
>> slightly more complex enqueue/dequeue operations.  The current version
>> will do unnecessary promotions but it was felt this was more than
>> compensated for by the lower enqueue/dequeue costs.  We'll see how a
>> more sophisticated version goes in terms of trade offs.
> 
> Would this affect the current, nearly perfect, spa_no_frills rr-behaviour w/ 
> its ability to circumvent the timeslice problem when setting promo to max?

No, I'd leave those controls in there.

> 
>>>> This is one good reason not to use spa_no_frills on
>>>> production systems.
>>> spa_ebs is great, but rather bursty.  Even setting max_ia_bonus=0
>>> doesn't fix that.   Is there a way to smooth it like spa_no_frills?
>> The principal determinant would be the smoothness of the yardstick.
>> This is supposed to represent the task with the highest (recent) CPU
>> usage rate per share and is used to determine how fairly CPU is being
>> distributed among the currently active tasks.  Tasks are given a
>> priority based on how their CPU usage rate per share compares to this
>> yardstick.  This means that as the system load and/or type of task
>> running changes the priorities of the tasks can change dramatically.
>>
>> Is the burstiness that you're seeing just in the observed priorities or
>> is it associated with behavioural burstiness as well?
> 
> It's behavioural, exhibited in a choking style, like a jumpy mouse move 
> during ia boosts.

Yeah, I just tried it on my machine with the same results.  It used to 
behave quite well so I must have broken something recently.  I've been 
trying different things for IA bonus calculations.

BTW I've increased the smoothing of my rate statistics and that should 
help smooth scheduling as a whole.  It used to average a tasks behaviour 
over its last 10 cycles but now it does it over 44.  Plus I've moved 
initial_time_slice as discussed.  I'll post patches for 2.6.17-rc1-mm2 
shortly.

> 
>>>> Perhaps you should consider creating a child
>>>> scheduler on top of it that meets your needs?
>>> Perhaps.
>> Good.  I've been hoping that other interested parties might be
>> encouraged by the small interface to SPA children to try different ideas
>> for scheduling.
> 
> Is there a no-op child skeleton available?

No.  But I could create one.

> 
>> One thing that could be played with here is to vary the time slice based
>> on the priority.  This would be in the opposite direction to the normal
>> scheduler with higher priority tasks (i.e. those with lower prio values)
>> getting smaller time slices.  The rationale being:
>>
>> 1. stop tasks that have been given large bonuses from shutting out other
>> tasks for too long, and
>> 2. reduce the context switch rate for tasks that haven't received bonuses.
>>
>> Because tasks that get large bonuses will have short CPU bursts they
>> should not be adversely effected (if this is done properly) as they will
>> (except in exceptional circumstances such as a change in behaviour)
>> surrender the CPU voluntarily before their reduced time slice has
>> expired.  Imaginative use of the available statistics could make this
>> largely automatic but there would be a need to be aware that the
>> statistics can be distorted by the shorter time slices.
>>
>> On the other hand, giving tasks without bonuses longer time slices
>> shouldn't adversely effect interactive performance as the interactive
>> tasks will (courtesy of their bonuses) preempt them.
> 
> I couldn't agree more.  Tackling the problem on both fronts (prio/tslice) may 
> give us more control, which could result in a more appropriate / fairer / 
> smoother scheduler.

"Hedging one's bets" as punters would say.

> 
> Thanks!

My pleasure.

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce