Messages in this thread | | | Date | Tue, 7 Mar 2006 17:11:34 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] mm: yield during swap prefetching |
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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? - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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