Messages in this thread | | | Date | Tue, 27 Sep 2011 18:06:48 +0200 | From | Johannes Weiner <> | Subject | Re: [PATCH v2 -mm] limit direct reclaim for higher order allocations |
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On Tue, Sep 27, 2011 at 10:52:46AM -0400, Rik van Riel wrote: > When suffering from memory fragmentation due to unfreeable pages, > THP page faults will repeatedly try to compact memory. Due to > the unfreeable pages, compaction fails. > > Needless to say, at that point page reclaim also fails to create > free contiguous 2MB areas. However, that doesn't stop the current > code from trying, over and over again, and freeing a minimum of > 4MB (2UL << sc->order pages) at every single invocation. > > This resulted in my 12GB system having 2-3GB free memory, a > corresponding amount of used swap and very sluggish response times. > > This can be avoided by having the direct reclaim code not reclaim > from zones that already have plenty of free memory available for > compaction. > > If compaction still fails due to unmovable memory, doing additional > reclaim will only hurt the system, not help. > > Signed-off-by: Rik van Riel <riel@redhat.com> > > --- > -v2: shrink_zones now uses the same thresholds as used by compaction itself, > not only is this conceptually nicer, it also results in kswapd doing > some actual work; before all the page freeing work was done by THP > allocators, I seem to see fewer application stalls after this change. > > mm/vmscan.c | 10 ++++++++++ > 1 files changed, 10 insertions(+), 0 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index b7719ec..117eb4d 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -2083,6 +2083,16 @@ static void shrink_zones(int priority, struct zonelist *zonelist, > continue; > if (zone->all_unreclaimable && priority != DEF_PRIORITY) > continue; /* Let kswapd poll it */ > + if (COMPACTION_BUILD) { > + /* > + * If we already have plenty of memory free > + * for compaction, don't free any more. > + */ > + if (sc->order > PAGE_ALLOC_COSTLY_ORDER && > + (compaction_suitable(zone, sc->order) || > + compaction_deferred(zone))) > + continue; > + }
I don't think the comment is complete in combination with the check for order > PAGE_ALLOC_COSTLY_ORDER, as compaction is invoked for all non-zero orders.
But the traditional behaviour does less harm if the orders are small and your problem was triggered by THP allocations, so I agree with the code itself.
Acked-by: Johannes Weiner <jweiner@redhat.com>
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