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    SubjectRe: [PATCH 4/4] mm: vmscan: Only read new_classzone_idx from pgdat when reclaiming successfully
    On 06/24/2011 10:44 AM, Mel Gorman wrote:
    > During allocator-intensive workloads, kswapd will be woken frequently
    > causing free memory to oscillate between the high and min watermark.
    > This is expected behaviour. Unfortunately, if the highest zone is
    > small, a problem occurs.
    > When balance_pgdat() returns, it may be at a lower classzone_idx than
    > it started because the highest zone was unreclaimable. Before checking
    > if it should go to sleep though, it checks pgdat->classzone_idx which
    > when there is no other activity will be MAX_NR_ZONES-1. It interprets
    > this as it has been woken up while reclaiming, skips scheduling and
    > reclaims again. As there is no useful reclaim work to do, it enters
    > into a loop of shrinking slab consuming loads of CPU until the highest
    > zone becomes reclaimable for a long period of time.
    > There are two problems here. 1) If the returned classzone or order is
    > lower, it'll continue reclaiming without scheduling. 2) if the highest
    > zone was marked unreclaimable but balance_pgdat() returns immediately
    > at DEF_PRIORITY, the new lower classzone is not communicated back to
    > kswapd() for sleeping.
    > This patch does two things that are related. If the end_zone is
    > unreclaimable, this information is communicated back. Second, if
    > the classzone or order was reduced due to failing to reclaim, new
    > information is not read from pgdat and instead an attempt is made to go
    > to sleep. Due to this, it is also necessary that pgdat->classzone_idx
    > be initialised each time to pgdat->nr_zones - 1 to avoid re-reads
    > being interpreted as wakeups.
    > Reported-and-tested-by: Pádraig Brady<>
    > Signed-off-by: Mel Gorman<>

    Acked-by: Rik van Riel <>

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