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    SubjectRe: [PATCH 0/4] Stop kswapd consuming 100% CPU when highest zone is small
    On Thu, Jul 21, 2011 at 12:24 PM, Minchan Kim <minchan.kim@gmail.com> wrote:
    > On Thu, Jul 21, 2011 at 05:09:59PM +0100, Mel Gorman wrote:
    >> 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.
    >
    > Of course, it's not equal with sleep but other task should get CPU and conusme their time slice
    > So we should never see 100% CPU consumption of kswapd.
    > No?

    If the rest of the system is idle, then kswapd will happily use 100%
    CPU. (Or on a multi-core system, kswapd will use close to 100% of one
    CPU even if another task is using the other one. This is bad enough
    on a desktop, but on a laptop you start to notice when your battery
    dies.)

    --Andy

    >
    >>
    >> --
    >> Mel Gorman
    >> SUSE Labs
    >
    > --
    > Kind regards,
    > Minchan Kim
    >
    --
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