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    SubjectRe: [PATCH] mm: Implement swap prefetching tweaks
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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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