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    SubjectRe: swapping and the value of /proc/sys/vm/swappiness
    On Wed, Sep 15, 2004 at 08:53:21AM +1000, Con Kolivas wrote:
    > Marcelo Tosatti wrote:
    > >On Tue, Sep 14, 2004 at 11:31:53AM -0700, Florin Andrei wrote:
    > >
    > >>On Mon, 2004-09-06 at 16:27, Andrew Morton wrote:
    > >>
    > >>>Con Kolivas <kernel@kolivas.org> wrote:
    > >>
    > >>>>The change was not deliberate but there have been some other people
    > >>>>report significant changes in the swappiness behaviour as well (see
    > >>>>archives). It has usually been of the increased swapping variety
    > >>>>lately. It has been annoying enough to the bleeding edge desktop users
    > >>>>for a swag of out-of-tree hacks to start appearing (like mine).
    > >>>
    > >>>All of which is largely wasted effort.
    > >>
    > >>>From a highly-theoretical, ivory-tower perspective, maybe; i am not the
    > >>one to pass judgement.
    > >>>From a realistic, "fix it 'cause it's performing worse than MSDOS
    > >>without a disk cache" perspective, definitely not true.
    > >>
    > >>I've found a situation where the vanilla kernel has a behaviour that
    > >>makes no sense:
    > >>
    > >>http://marc.theaimsgroup.com/?l=linux-kernel&m=109237941331221&w=2
    > >>http://marc.theaimsgroup.com/?l=linux-kernel&m=109237959719868&w=2
    > >>http://marc.theaimsgroup.com/?l=linux-kernel&m=109238126314192&w=2
    > >>
    > >>A patch by Con Kolivas fixed it:
    > >>
    > >>http://marc.theaimsgroup.com/?l=linux-kernel&m=109410526607990&w=2
    > >>
    > >>I cannot offer more details, i have no time for experiments, i just need
    > >>a system that works. The vanilla kernel does not.
    > >
    > >
    > >Have you tried to decrease the value of /proc/sys/vm/swappiness
    > >to say 30 and see what you get?
    > >
    > >Andrew's point is that we should identify the problem - Con's patch
    > >rewrites swapping policy.
    >
    > I already answered this. That hard swappiness patch does not really
    > rewrite swapping policy. It identifies exactly what has changed because
    > it does not count "distress in the swap tendency". Therefore if the
    > swappiness value is the same, the mapped ratio is the same (in the
    > workload) yet the vm is swappinig more, it is getting into more
    > "distress". The mapped ratio is the same but the "distress" is for some
    > reason much higher in later kernels, meaning the priority of our
    > scanning is getting more and more intense. This should help direct your
    > searches.

    > These are the relevant lines of code _from mainline_:
    >
    > distress = 100 >> zone->prev_priority
    > mapped_ratio = (sc->nr_mapped * 100) / total_memory;
    > swap_tendency = mapped_ratio / 2 + distress + vm_swappiness
    > if (swap_tendency >= 100)
    > - reclaim_mapped = 1;
    >
    >
    > That hard swappiness patch effectively made "distress == 0" always.

    OK.

    So isnt it true that decreasing vm_swappiness should compensate
    distress and have the same effect of your patch?

    To be fair I'm just arguing, haven't really looked at the code.



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