linux-kernel - Re: kswapd skips compaction if reclaim order drops to zero?

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Date:	Thu, 15 Aug 2013 14:56:27 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Hillf Danton <dhillf@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>
Subject: Re: kswapd skips compaction if reclaim order drops to zero?

On Thu, Aug 15, 2013 at 10:41:39PM +0900, Minchan Kim wrote:
> Hey Mel,
> 
> On Thu, Aug 15, 2013 at 11:47:27AM +0100, Mel Gorman wrote:
> > On Thu, Aug 15, 2013 at 06:02:53PM +0800, Hillf Danton wrote:
> > > If the allocation order is not high, direct compaction does nothing.
> > > Can we skip compaction here if order drops to zero?
> > > 
> > 
> > If the allocation order is not high then
> > 
> > pgdat_needs_compaction == (order > 0) == false == no calling compact_pdatt
> > 
> > In the case where order is reset to 0 due to fragmentation then it does
> > call compact_pgdat but it does no work due to the cc->order check in
> > __compact_pgdat.
> > 
> 
> I am looking at mmotm-2013-08-07-16-55 but couldn't find cc->order
> check right before compact_zone in __comact_pgdat.
> Could you pinpoint code piece?
> 

Thanks, I screwed up as that check happens too late. However, it still
ends up not mattering because it does this

compact_pgdat
  -> __compact_pgdat
    -> compact_zone
      -> compaction_suitable

For order == 0, compaction_suitable will return either COMPACT_SKIPPED
(if the watermarks are not met) and COMPACT_PARTIAL otherwise. Either
way, compaction doesn't run.

-- 
Mel Gorman
SUSE Labs
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