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SubjectRe: dead mem walking ;-)


On Thu, 12 Jul 2001, Dirk Wetter wrote:

> On Thu, 12 Jul 2001, Marcelo Tosatti wrote:
>
>
> > > a while before the jobs were submitted i did "readprofile | sort -nr | head -10":
> > > 296497 total 0.3442
> > > 295348 default_idle 5679.7692
> > > 300 __rdtsc_delay 10.7143
> > > 215 si_swapinfo 1.2500
> > > 138 do_softirq 1.0147
> > > 107 printk 0.2816
> > > 28 do_wp_page 0.0272
> > > 17 schedule 0.0117
> > > 10 tcp_get_info 0.0077
> > > 10 filemap_nopage 0.0073
> > >
> > > the same after i was able to kill the jobs (see below):
> > >
> > > 836552 total 0.9710
> > > 458757 default_idle 8822.2500
> > > 361961 __get_swap_page 665.3695
> > > 6629 si_swapinfo 38.5407
> > > 1655 do_anonymous_page 5.3734
> > > 760 file_read_actor 3.0645
> > > 652 statm_pgd_range 1.6633
> > > 592 do_softirq 4.3529
> > > 498 skb_copy_bits 0.5845
> > > 302 __rdtsc_delay 10.7857
> >
> >
> > Ok, I've seen that before. __get_swap_page() is horribly innefficient.
>
> :-(
>
> > The system is _not_ swaping out data, though. Its just aging the
> > pte's and allocating swap.
>
> with that jobs it looks to me that swap allocation shouldn't be
> neccessary? total of all pages should have been below the physcial mem
> size.

Well, the kernel tries to keep a given amount of pages in a "deactivated"
state (deactivated = ready-to-free) so it can keep a low amount of actual
free pages (amongs other benefits).

Anonymous pages (from your processes) need their space _allocated_ on swap
before they can be aged and possibly written out to swap and freed later.

If you look at the "inactive_target" field on /proc/meminfo you will see
how much data the kernel is trying to keep deactivated.

> > And that is what is eating the system performance.
>
> does it bring up the load up to 30 and make the machine unusable?
> (kswapd was also sometimes in the top-list of CPU hogs, but since i
> sorted it by memory...)

Yes. Obviously that should not happen.

What you're seeing _is_ a problem.

> > <snip>
> >
> > Can you please show us the output of /proc/meminfo when the system is
> > behaving badly ?
>
> hold on, we set s.th. up....

Ok, thanks.

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