Messages in this thread | | | From | Con Kolivas <> | Subject | Re: [PATCH] mm: yield during swap prefetching | Date | Wed, 8 Mar 2006 12:12:02 +1100 |
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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.
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