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

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Message-ID: <20130815141004.GD8437@gmail.com>
Date:	Thu, 15 Aug 2013 23:10:04 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Mel Gorman <mgorman@...e.de>
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 02:56:27PM +0100, Mel Gorman wrote:
> 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.

In compaction_suitable, it could pass first zone_watermark_ok
but failed second zone_watermark_ok while fragindex is -1000
so compaction could run.

And we shouldn't depend on such coincidence.


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