Messages in this thread | | | From | Con Kolivas <> | Subject | Re: [PATCH] mm: Implement swap prefetching tweaks | Date | Sat, 11 Mar 2006 18:24:05 +1100 |
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On Saturday 11 March 2006 17:00, Mike Galbraith wrote: > On Sat, 2006-03-11 at 16:50 +1100, Con Kolivas wrote: > > On Saturday 11 March 2006 16:33, Mike Galbraith wrote: > > > On Sat, 2006-03-11 at 14:50 +1100, Con Kolivas wrote: > > > > On Saturday 11 March 2006 09:35, Andrew Morton wrote: > > > > > Con Kolivas <kernel@kolivas.org> wrote: > > > > > > + /* > > > > > > + * get_page_state is super expensive so we only perform it > > > > > > every + * SWAP_CLUSTER_MAX prefetched_pages. > > > > > > > > > > nr_running() is similarly expensive btw. > > > > > > > > Yes which is why I do it just as infrequently as get_page_state. > > > > > > > > > > * We also test if we're the only > > > > > > + * task running anywhere. We want to have as little impact on > > > > > > all + * resources (cpu, disk, bus etc). As this iterates over > > > > > > every cpu + * we measure this infrequently. > > > > > > + */ > > > > > > + if (!(sp_stat.prefetched_pages % SWAP_CLUSTER_MAX)) { > > > > > > + unsigned long cpuload = nr_running(); > > > > > > + > > > > > > + if (cpuload > 1) > > > > > > + goto out; > > > > > > > > > > Sorry, this is just wrong. If swap prefetch is useful then it's > > > > > also useful if some task happens to be sitting over in the corner > > > > > calculating pi. > > > > > > > > > > What's the actual problem here? Someone's 3d game went blippy? > > > > > Why? How much? Are we missing a cond_resched()? > > > > > > > > No, it's pretty easy to reproduce, kprefetchd sits there in > > > > uninterruptible sleep with one cpu on SMP pegged at 100% iowait due > > > > to it. This tends to have noticeable effects everywhere on HT or SMP. > > > > On UP the yielding helped it but even then it still causes blips. How > > > > much? Well to be honest it's noticeable a shipload. Running a game, > > > > any game, that uses 100% (and most fancy games do) causes stuttering > > > > on audio, pauses and so on. This is evident on linux native games, > > > > games under emulators or qemu and so on. That iowait really hurts, > > > > and tweaking just priority doesn't help it in any way. > > > > > > That doesn't really make sense to me. If a task can trigger audio > > > dropout and stalls by sleeping, we have a serious problem. In your > > > SMP/HT case, I'd start crawling over the load balancing code. I can't > > > see how trivial CPU with non-saturated IO can cause dropout in the UP > > > case either. Am I missing something? > > > > Clearly you, me and everyone else is missing something. I see it with > > each task bound to one cpu with cpu affinity so it's not a balancing > > issue. Try it yourself if you can instead of not believing me. Get a big > > dd reader (virtually no cpu and all io wait sleep) on one cpu and try and > > play a game on the other cpu. It dies rectally. > > I said it didn't make sense to me, not that I didn't believe you. If I > had a real SMP box, I would look into it, but all I have is HT.
No doubt it would be better on an SMP box. The norm is, however, for all these multi-core, multi-threading cpus to be more common than real SMP and they all share varying amounts of their resources.
> If you're creating a lot of traffic, I can see it causing problems. I > was under the impression that you were doing minimal IO and absolutely > trivial CPU. That's what didn't make sense to me to be clear.
A lot of cpu would be easier to handle; it's using absolutely miniscule amounts of cpu. The IO is massive though (and seeky in nature), and reading from a swap partition seems particularly expensive in this regard.
Cheers, Con - 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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