Re: [PATCH] mm: yield during swap prefetching

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

Con Kolivas wrote:
> On Thursday 09 March 2006 19:57, Helge Hafting wrote:
> 
>>Con Kolivas wrote:
>>
>>>On Wed, 8 Mar 2006 12:11 pm, Andrew Morton wrote:
>>>
>>>>but, but.  If prefetching is prefetching stuff which that game will soon
>>>>use then it'll be an aggregate improvement.  If prefetch is prefetching
>>>>stuff which that game _won't_ use then prefetch is busted.  Using yield()
>>>>to artificially cripple kprefetchd is a rather sad workaround isn't it?
>>>
>>>It's not the stuff that it prefetches that's the problem; it's the disk
>>>access.
>>
>>Well, seems you have some sorry kind of disk driver then?
>>An ide disk not using dma?
>>
>>A low-cpu task that only abuses the disk shouldn't make an impact
>>on a 3D game that hogs the cpu only.  Unless the driver for your
>>harddisk is faulty, using way more cpu than it need.
>>
>>Use hdparm, check the basics:
>>unmaksirq=1, using_dma=1, multcount is some positive number,
>>such as 8 or 16, readahead is some positive number.
>>Also use hdparm -i and verify that the disk is using some
>>nice udma mode.  (too old for that, and it probably isn't worth
>>optimizing this for...)
>>
>>Also make sure the disk driver isn't sharing an irq with the
>>3D card.
>>
>>Come to think of it, if your 3D game happens to saturate the
>>pci bus for long times, then disk accesses might indeed
>>be noticeable as they too need the bus.  Check if going to
>>a slower dma mode helps - this might free up the bus a bit.
> 
> 
> Thanks for the hints. 
> 
> However I actually wrote the swap prefetch code and this is all about changing 
> its behaviour to make it do what I want. The problem is that nice 19 will 
> give it up to 5% cpu in the presence of a nice 0 task when I really don't 
> want swap prefetch doing anything.

I'm working on a patch to add soft and hard CPU rate caps to the
scheduler and the soft caps may be useful for what you're trying to do.
  They are a generalization of your SCHED_BATCH implementation in
staircase (which would have been better called SCHED_BACKGROUND :-)
IMHO) in that a task with a soft cap will only use more CPU than that
cap if it (the cpu) would otherwise go unused.  The main difference
between this mechanism and staircase's SCHED_BATCH mechanism is that you
can specify how much (as parts per thousand of a CPU) the task can use
instead of just being background or not background.  With the soft cap
set to zero the effect would be essentially the same.

> Furthermore because it is constantly 
> waking up from sleep (after disk activity) it is always given lower latency 
> scheduling than a fully cpu bound nice 0 task - this is normally appropriate 
> behaviour. Yielding regularly works around that issue. 
> 
> Ideally taking into account cpu usage and only working below a certain cpu 
> threshold may be the better mechanism and it does appear this would be more 
> popular. It would not be hard to implement, but does add yet more code to an 
> increasingly complex heuristic used to detect "idleness". I am seriously 
> considering it.

See above re CPU rate soft caps.  I'm holding off on submitting this
patch for consideration until the current scheduler modifications being
tested in -mm have had time to settle.

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

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

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