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    SubjectRe: balance_pgdat(): where is total_scanned ever updated?
    > > > > I had a patch which fixes it in -mm for a while.  It does increase the
    > > > > number of pages which are reclaimed via direct reclaim and decreases the
    > > > > number of pages which are reclaimed by kswapd. As one would expect from
    > > > > throttling kswapd. This seems undesirable.
    > > >
    > > > Hi Andrew,
    > > >
    > > > Do you have any numbers to backup the claim "It does increase the
    > > > number of pages which are reclaimed via direct reclaim and decreases the
    > > > number of pages which are reclaimed by kswapd", please?
    > >
    > > Run a workload and watch /proc/vmstat. iirc, the one-line total_scanned
    > > fix takes the kswapd-vs-direct reclaim rate from 1:1 to 1:3 or thereabouts.
    >
    > You're talking about laptop_mode ONLY, then?

    No, not at all.

    If we restore the total_scanned logic then kswapd will throttle itself, as
    designed. Regardless of laptop_mode. I did that, and monitored the page
    scanning and reclaim rates under various workloads. I observed that with
    the fix in place, kswapd performed less page reclaim and direct-reclaim
    performed more reclaim. And I wasn't able to demonstrate any benchmark
    improvements with the fix in place, so things are left as they are.

    > How can that have any effect if may_writepage is ignored if !laptop_mode?

    This is to do with kswapd throttling. If we put kswapd to sleep more
    often, it does less scanning and reclaiming.

    > About /proc/vmstat - each output is huge - do you actually read those?

    yup.

    cat /proc/vmstat > /tmp/1
    run workload
    cat /proc/vmstat > /tmp/2
    analyse /tmp/1 and /tmp/2

    > We need a vmstat like tool for that information to be readable.

    Would be nice.

    > > > Because linux-2.6.10-rc1-mm2 (and 2.6.9) completly ignores sc->may_writepage
    > > > under normal operation, its only used when laptop_mode is on:
    > > >
    > > > if (laptop_mode && !sc->may_writepage)
    > > > goto keep_locked;
    > > >
    > > > Is this intentional ???
    > >
    > > yup. In laptop mode we try to scan further to find a clean page rather
    > > than spinning up the disk for a writepage.
    >
    > It might be interesting to use sc->may_writepage independantly of
    > laptop mode (ie make kswapd only writeout pages if the reclaim ratio
    > is low).

    sure.

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