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    SubjectRe: [PATCH 3/3] mm: page allocator: Reconsider zones for allocation after direct reclaim
    On Thu, Jul 14, 2011 at 12:20:38PM +0900, KOSAKI Motohiro wrote:
    > (2011/07/13 20:10), Mel Gorman wrote:
    > > On Wed, Jul 13, 2011 at 09:42:39AM +0900, KOSAKI Motohiro wrote:
    > >> (2011/07/11 22:01), Mel Gorman wrote:
    > >>> With zone_reclaim_mode enabled, it's possible for zones to be considered
    > >>> full in the zonelist_cache so they are skipped in the future. If the
    > >>> process enters direct reclaim, the ZLC may still consider zones to be
    > >>> full even after reclaiming pages. Reconsider all zones for allocation
    > >>> if direct reclaim returns successfully.
    > >>>
    > >>> Signed-off-by: Mel Gorman <>
    > >>
    > >> Hmmm...
    > >>
    > >> I like the concept, but I'm worry about a corner case a bit.
    > >>
    > >> If users are using cpusets/mempolicy, direct reclaim don't scan all zones.
    > >> Then, zlc_clear_zones_full() seems too aggressive operation.
    > >
    > > As the system is likely to be running slow if it is in direct reclaim
    > > that the complexity of being careful about which zone was cleared was
    > > not worth it.
    > >
    > >> Instead, couldn't we turn zlc->fullzones off from kswapd?
    > >>
    > >
    > > Which zonelist should it clear (there are two) and when should it
    > > happen? If it clears it on each cycle around balance_pgdat(), there
    > > is no guarantee that it'll be cleared between when direct reclaim
    > > finishes and an attempt is made to allocate.
    > Hmm..
    > Probably I'm now missing the point of this patch. Why do we need
    > to guarantee tightly coupled zlc cache and direct reclaim?

    Because direct reclaim may free enough memory such that the zlc cache
    stating the zone is full is wrong.

    > IIUC,
    > zlc cache mean "to avoid free list touch if they have no free mem".
    > So, any free page increasing point is acceptable good, I thought.
    > In the other hand, direct reclaim finishing has no guarantee to
    > zones of zonelist have enough free memory because it has bailing out logic.

    It has no guarantee but there is a reasonable expectation that direct
    reclaim will free some memory that means we should reconsider the
    zone for allocation.

    > So, I think we don't need to care zonelist, just kswapd turn off
    > their own node.

    I don't understand what you mean by this.

    > And, just curious, If we will have a proper zlc clear point, why
    > do we need to keep HZ timeout?

    Yes because we are not guaranteed to call direct reclaim either. Memory
    could be freed by a process exiting and I'd rather not add cost to
    the free path to find and clear all zonelists referencing the zone the
    page being freed belongs to.

    Mel Gorman
    SUSE Labs

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