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    SubjectRe: [patch 04/11] mm: memcg: per-priority per-zone hierarchy scan generations
    On Tue 13-09-11 13:03:01, Johannes Weiner wrote:
    > On Tue, Sep 13, 2011 at 07:27:59PM +0900, KAMEZAWA Hiroyuki wrote:
    > > On Mon, 12 Sep 2011 12:57:21 +0200
    > > Johannes Weiner <jweiner@redhat.com> wrote:
    > >
    > > > Memory cgroup limit reclaim currently picks one memory cgroup out of
    > > > the target hierarchy, remembers it as the last scanned child, and
    > > > reclaims all zones in it with decreasing priority levels.
    > > >
    > > > The new hierarchy reclaim code will pick memory cgroups from the same
    > > > hierarchy concurrently from different zones and priority levels, it
    > > > becomes necessary that hierarchy roots not only remember the last
    > > > scanned child, but do so for each zone and priority level.
    > > >
    > > > Furthermore, detecting full hierarchy round-trips reliably will become
    > > > crucial, so instead of counting on one iterator site seeing a certain
    > > > memory cgroup twice, use a generation counter that is increased every
    > > > time the child with the highest ID has been visited.
    > > >
    > > > Signed-off-by: Johannes Weiner <jweiner@redhat.com>
    > >
    > > I cannot image how this works. could you illustrate more with easy example ?
    >
    > Previously, we did
    >
    > mem = mem_cgroup_iter(root)
    > for each priority level:
    > for each zone in zonelist:
    >
    > and this would reclaim memcg-1-zone-1, memcg-1-zone-2, memcg-1-zone-3
    > etc.
    >
    > The new code does
    >
    > for each priority level
    > for each zone in zonelist
    > mem = mem_cgroup_iter(root)
    >
    > but with a single last_scanned_child per memcg, this would scan
    > memcg-1-zone-1, memcg-2-zone-2, memcg-3-zone-3 etc, which does not
    > make much sense.
    >
    > Now imagine two reclaimers. With the old code, the first reclaimer
    > would pick memcg-1 and scan all its zones, the second reclaimer would
    > pick memcg-2 and reclaim all its zones. Without this patch, the first
    > reclaimer would pick memcg-1 and scan zone-1, the second reclaimer
    > would pick memcg-2 and scan zone-1, then the first reclaimer would
    > pick memcg-3 and scan zone-2. If the reclaimers are concurrently
    > scanning at different priority levels, things are even worse because
    > one reclaimer may put much more force on the memcgs it gets from
    > mem_cgroup_iter() than the other reclaimer. They must not share the
    > same iterator.
    >
    > The generations are needed because the old algorithm did not rely too
    > much on detecting full round-trips. After every reclaim cycle, it
    > checked the limit and broke out of the loop if enough was reclaimed,
    > no matter how many children were reclaimed from. The new algorithm is
    > used for global reclaim, where the only exit condition of the
    > hierarchy reclaim is the full roundtrip, because equal pressure needs
    > to be applied to all zones.

    Could you fold this into the patch description, please?

    --
    Michal Hocko
    SUSE Labs
    SUSE LINUX s.r.o.
    Lihovarska 1060/12
    190 00 Praha 9
    Czech Republic


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