linux-kernel - Re: Free memory never fully used, swapping

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Message-Id: <20101129100707.82A2.A69D9226@jp.fujitsu.com>
Date:	Mon, 29 Nov 2010 10:13:23 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	kosaki.motohiro@...fujitsu.com, Mel Gorman <mel@....ul.ie>,
	Simon Kirby <sim@...tway.ca>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: Free memory never fully used, swapping

> ok let me clarify, in the for-loop of balance_pgdat() we reclaim 32
> pages one time. but we have
> if (!all_zones_ok) {
> ...
> 		if (sc.nr_reclaimed < SWAP_CLUSTER_MAX)
> 			order = sc.order = 0;
> 
> 		goto loop_again;
> 	}
> only when sc.nr_reclaimed < SWAP_CLUSTER_MAX or priority < 0, we set
> order to 0. before this, we still use high order for zone_watermark_ok()
> and it will fail and we keep doing page reclaim. So in the proposed
> patch by you or Mel, checking the freed pages or order in kswapd() is
> later. so I suggest we check if there is enough free pages in
> balance_pgdat() and break high order allocation if yes.

Ok, got it. Thanks. But I think Mel's approach is more conservative. I don't
think a lot of order-0 pages are good sign to ignore high order shortage.

I think we should prevent overkill reclaim, but number of order-0 pages
are not related high order overkill.

My point is, many cheap device require high order GFP_ATOMIC allocation
and high order ignorerance may makes system unstabilization on laptop world.

At least, your patch need more conservative guard.


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