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    SubjectRe: swapping and the value of /proc/sys/vm/swappiness
    Marcelo Tosatti wrote:
    > 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.
    >
    > So isnt it true that decreasing vm_swappiness should compensate
    > distress and have the same effect of your patch?

    Nope. We swap large amounts with the wrong workload at swappiness==0
    where we wouldn't before at swappiness==60. ie there is no workaround
    possible without changing the code in some way.

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

    Thats cool ;)

    Cheers,
    Con
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