On Tue, Sep 27, 2011 at 06:06:48PM +0200, Johannes Weiner wrote:
> 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@xxxxxxxxxx>
> >
> > ---
> > -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.
FWIW, an incremental patch to explain the order check. What do you
think?
Signed-off-by: Johannes Weiner <jweiner@xxxxxxxxxx>
---
mm/vmscan.c | 10 ++++++++--
1 files changed, 8 insertions(+), 2 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 3817fa9..930085a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2068,8 +2068,14 @@ static void shrink_zones(int priority, struct zonelist *zonelist,
continue; /* Let kswapd poll it */
if (COMPACTION_BUILD) {
/*
- * If we already have plenty of memory free
- * for compaction, don't free any more.
+ * If we already have plenty of memory
+ * free for compaction, don't free any
+ * more. Even though compaction is
+ * invoked for any non-zero order,
+ * only frequent costly order
+ * reclamation is disruptive enough to
+ * become a noticable problem, like
+ * transparent huge page allocations.
*/
if (sc->order > PAGE_ALLOC_COSTLY_ORDER &&
(compaction_suitable(zone, sc->order) ||
--
1.7.6.4
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