From: Con Kolivas <kernel@kolivas.org>
To: Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH] mm: yield during swap prefetching
Date: Wed, 8 Mar 2006 12:19:21 +1100
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On Wed, 8 Mar 2006 12:12 pm, Con Kolivas wrote:
> On Wed, 8 Mar 2006 12:11 pm, Andrew Morton wrote:
> > Con Kolivas <kernel@kolivas.org> wrote:
> > > On Wed, 8 Mar 2006 11:05 am, Andrew Morton wrote:
> > > > Con Kolivas <kernel@kolivas.org> wrote:
> > > > > > yield() really sucks if there are a lot of runnable tasks.  And
> > > > > > the amount of CPU which that thread uses isn't likely to matter
> > > > > > anyway.
> > > > > >
> > > > > > I think it'd be better to just not do this.  Perhaps alter the
> > > > > > thread's static priority instead?  Does the scheduler have a knob
> > > > > > which can be used to disable a tasks's dynamic priority boost
> > > > > > heuristic?
> > > > >
> > > > > We do have SCHED_BATCH but even that doesn't really have the
> > > > > desired effect. I know how much yield sucks and I actually want it
> > > > > to suck as much as yield does.
> > > >
> > > > Why do you want that?
> > > >
> > > > If prefetch is doing its job then it will save the machine from a
> > > > pile of major faults in the near future.  The fact that the machine
> > > > happens to be running a number of busy tasks doesn't alter that. 
> > > > It's _worth_ stealing a few cycles from those tasks now to avoid
> > > > lengthy D-state sleeps in the near future?
> > >
> > > The test case is the 3d (gaming) app that uses 100% cpu. It never sets
> > > delay swap prefetch in any way so swap prefetching starts working. Once
> > > swap prefetching starts reading it is mostly in uninterruptible sleep
> > > and always wakes up on the active array ready for cpu, never expiring
> > > even with its miniscule timeslice. The 3d app is always expiring and
> > > landing on the expired array behind kprefetchd even though kprefetchd
> > > is nice 19. The practical upshot of all this is that kprefetchd does a
> > > lot of prefetching with 3d gaming going on, and no amount of priority
> > > fiddling stops it doing this. The disk access is noticeable during 3d
> > > gaming unfortunately. Yielding regularly means a heck of a lot less
> > > prefetching occurs and is no longer noticeable. When idle, yield()ing
> > > doesn't seem to adversely affect the effectiveness of the prefetching.
> >
> > 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.

I guess what I'm saying is there isn't enough information to delay swap 
prefetch when cpu usage is high which was my intention as well. Yielding has 
the desired effect without adding further accounting checks to swap_prefetch.

Cheers,
Con
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