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    SubjectRe: [PATCH 0/4] Stop kswapd consuming 100% CPU when highest zone is small
    On Fri, Jul 22, 2011 at 12:37:22AM +0900, Minchan Kim wrote:
    > On Fri, Jun 24, 2011 at 03:44:53PM +0100, Mel Gorman wrote:
    > > (Built this time and passed a basic sniff-test.)
    > >
    > > 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.
    > >
    > > This seems to happen most with recent sandybridge laptops but it's
    > > probably a co-incidence as some of these laptops just happen to have
    > > a small Normal zone. The reproduction case is almost always during
    > > copying large files that kswapd pegs at 100% CPU until the file is
    > > deleted or cache is dropped.
    > >
    > > The problem is mostly down to sleeping_prematurely() keeping kswapd
    > > awake when the highest zone is small and unreclaimable and compounded
    > > by the fact we shrink slabs even when not shrinking zones causing a lot
    > > of time to be spent in shrinkers and a lot of memory to be reclaimed.
    > >
    > > Patch 1 corrects sleeping_prematurely to check the zones matching
    > > the classzone_idx instead of all zones.
    > >
    > > Patch 2 avoids shrinking slab when we are not shrinking a zone.
    > >
    > > Patch 3 notes that sleeping_prematurely is checking lower zones against
    > > a high classzone which is not what allocators or balance_pgdat()
    > > is doing leading to an artifical believe that kswapd should be
    > > still awake.
    > >
    > > Patch 4 notes that when balance_pgdat() gives up on a high zone that the
    > > decision is not communicated to sleeping_prematurely()
    > >
    > > This problem affects 2.6.38.8 for certain and is expected to affect
    > > 2.6.39 and 3.0-rc4 as well. If accepted, they need to go to -stable
    > > to be picked up by distros and this series is against 3.0-rc4. I've
    > > cc'd people that reported similar problems recently to see if they
    > > still suffer from the problem and if this fixes it.
    > >
    >
    > Good!
    > This patch solved the problem.
    > But there is still a mystery.
    >
    > In log, we could see excessive shrink_slab calls.

    Yes, because shrink_slab() was called on each loop through
    balance_pgdat() even if the zone was balanced.


    > And as you know, we had merged patch which adds cond_resched where last of the function
    > in shrink_slab. So other task should get the CPU and we should not see
    > 100% CPU of kswapd, I think.
    >

    cond_resched() is not a substitute for going to sleep.

    --
    Mel Gorman
    SUSE Labs


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