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SubjectRe: [patch] voluntary-preempt-2.6.9-rc1-bk12-R6
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On Thu, 2004-09-09 at 15:29, Ingo Molnar wrote:
> * Lee Revell <rlrevell@joe-job.com> wrote:
>
> > I get these latencies when I cause the machine to swap by compiling a
> > kernel with make -j32. They get bigger as the machine gets further
> > into swap.
> >
> > Every 2.0s: head -60 /proc/latency_trace Wed Sep 8 02:51:40 2004
> >
> > preemption latency trace v1.0.6 on 2.6.9-rc1-bk12-VP-R6
> > --------------------------------------------------
> > latency: 605 us, entries: 5 (5) [VP:1 KP:1 SP:1 HP:1 #CPUS:1]
> > -----------------
> > | task: kswapd0/35, uid:0 nice:0 policy:0 rt_prio:0
> > -----------------
> > => started at: get_swap_page+0x23/0x490
> > => ended at: get_swap_page+0x13f/0x490
> > =======>
> > 00000001 0.000ms (+0.606ms): get_swap_page (add_to_swap)
> > 00000001 0.606ms (+0.000ms): sub_preempt_count (get_swap_page)
> > 00000001 0.606ms (+0.000ms): update_max_trace (check_preempt_timing)
> > 00000001 0.606ms (+0.000ms): _mmx_memcpy (update_max_trace)
> > 00000001 0.607ms (+0.000ms): kernel_fpu_begin (_mmx_memcpy)
>
> yep, the get_swap_page() latency. I can easily trigger 10+ msec
> latencies on a box with alot of swap by just letting stuff swap out. I
> had a quick look but there was no obvious way to break the lock. Maybe
> Andrew has better ideas? get_swap_page() is pretty stupid, it does a
> near linear search for a free slot in the swap bitmap - this not only is
> a latency issue but also an overhead thing as we do it for every other
> page that touches swap.
>
> rationale: this is pretty much the only latency that we still having
> during heavy VM load and it would Just Be Cool if we fixed this final
> one. audio daemons and apps like jackd use mlockall() so they are not
> affected by swapping.
>

I believe Scott Wood suggested a fix back when I first reported this,
have to check my mailbox. Scott?

Lee


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