Re: [PATCH v2 8/9] mm/compaction: don't use higher order freepage than compaction aims at

Joonsoo Kim
2015-09-30T08:37:29Z

On Fri, Sep 25, 2015 at 01:41:48PM +0200, Vlastimil Babka wrote:
> On 08/24/2015 04:19 AM, Joonsoo Kim wrote:
> > Purpose of compaction is to make high order page. To achive this purpose,
> > it is the best strategy that compaction migrates contiguous used pages
> > to fragmented unused freepages. Currently, freepage scanner don't
> > distinguish whether freepage is fragmented or not and blindly use
> > any freepage for migration target regardless of freepage's order.
> >
> > Using higher order freepage than compaction aims at is not good because
> > what we do here is breaking high order freepage at somewhere and migrating
> > used pages from elsewhere to this broken high order freepages in order to
> > make new high order freepage. That is just position change of high order
> > freepage.
> >
> > This is useless effort and doesn't help to make more high order freepages
> > because we can't be sure that migrating used pages makes high order
> > freepage. So, this patch makes freepage scanner only uses the ordered
> > freepage lower than compaction order.
>
> How often does this happen? If there's a free page of the order we need, then we
> are done compacting anyway, no? Or is this happening because of the current
> high-order watermark checking implementation? It would be interesting to measure
> how often this skip would trigger. Also watermark checking should change with
> Mel's patchset and then this patch shouldn't be needed?

Yes, you are right. This would be happening because of the current
high-order watermakr checking implementation and Mel's patchset will
solve it and if it is merged, this patch isn't needed.

>
> > Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> > ---
> > mm/compaction.c | 15 +++++++++++++++
> > 1 file changed, 15 insertions(+)
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index ca4d6d1..e61ee77 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -455,6 +455,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> > unsigned long flags = 0;
> > bool locked = false;
> > unsigned long blockpfn = *start_pfn;
> > + unsigned long freepage_order;
> >
> > cursor = pfn_to_page(blockpfn);
> >
> > @@ -482,6 +483,20 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
> > if (!PageBuddy(page))
> > goto isolate_fail;
> >
> > + if (!strict && cc->order != -1) {
> > + freepage_order = page_order_unsafe(page);
> > +
> > + if (freepage_order > 0 && freepage_order < MAX_ORDER) {
> > + /*
> > + * Do not use high order freepage for migration
> > + * taret. It would not be beneficial for
> > + * compaction success rate.
> > + */
> > + if (freepage_order >= cc->order)
>
> It would be better to skip the whole freepage_order.

Okay.

Thanks.

>
> > + goto isolate_fail;
> > + }
> > + }
> > +
> > /*
> > * If we already hold the lock, we can skip some rechecking.
> > * Note that if we hold the lock now, checked_pageblock was
> >
>
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