Messages in this thread | | | Subject | Re: [PATCH] mm: Implement swap prefetching tweaks | From | Mike Galbraith <> | Date | Sat, 11 Mar 2006 07:11:35 +0100 |
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On Sat, 2006-03-11 at 16:58 +1100, Con Kolivas wrote: > On Saturday 11 March 2006 16:50, 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 happen to have a tool to instrument this as you're probably aware > (interbench). Here is an old log I found of this.:
Yeah, I have a copy. Interpreting the results isn't necessarily easy though, just as any other benchmark.
> > --- Benchmarking simulated cpu of Gaming in the presence of simulated --- > Load Latency +/- SD (ms) Max Latency % Desired CPU > None 0 +/- 0 0 100 > Write 36.5 +/- 103 966 73.3 > Read 17.2 +/- 22.9 244 85.3 > > Note the max latency being massive and desired cpu dropping. This is on a HT > machine.
I wonder what that would look like with two real CPUs.
-Mike
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