Re: [PATCH] mm: yield during swap prefetching

Con Kolivas
2006-03-08T00:50:47Z

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.

Cheers,
Con