Re: [PATCH] mm: yield during swap prefetching

Andrew Morton
2006-03-08T01:09:24Z

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?