linux-kernel - Re: [PATCH 55 of 66] select CONFIG_COMPACTION if TRANSPARENT

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Message-ID: <20101214094556.GF13914@csn.ul.ie>
Date:	Tue, 14 Dec 2010 09:45:56 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>, bpicco@...hat.com,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Chris Mason <chris.mason@...cle.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 55 of 66] select CONFIG_COMPACTION if
	TRANSPARENT_HUGEPAGE enabled

On Thu, Dec 09, 2010 at 08:04:07PM +0100, Andrea Arcangeli wrote:
> On Thu, Nov 18, 2010 at 04:22:45PM +0000, Mel Gorman wrote:
> > Just to confirm - by hang, you mean grinds to a slow pace as opposed to
> > coming to a complete stop and having to restart?
> 
> Hmm it's like if you're gigabytes in swap and apps hangs for a while
> and system is not really usable and it swaps for most new memory
> allocations despite there's plenty of memory free, but it's not a
> deadlock of course.
> 

Ok, but it's likely to be kswapd being very aggressive because it's
woken up frequently and tries to balance all zones. Once it's not
deadlocking entirely, there isn't a more fundamental bug hiding in there
somewhere.

> BTW, alternatively I could:
> 
>  unsigned long transparent_hugepage_flags __read_mostly =
>         (1<<TRANSPARENT_HUGEPAGE_FLAG)|
> +#ifdef CONFIG_COMPACTION
> +       (1<<TRANSPARENT_HUGEPAGE_DEFRAG_FLAG)|
> +#endif
>         (1<<TRANSPARENT_HUGEPAGE_DEFRAG_KHUGEPAGED_FLAG);
> 
> That would adds GFP_ATOMIC to THP allocation if compaction wasn't
> selected,

With GFP_NO_KSWAPD, it would stop trashing I suspect the success rate
would be extremely low as nothing will be defragmenting memory.

> but I think having compaction enabled diminish the risk of
> misconfigured kernels leading to unexpected measurements and behavior,
> so I feel much safer to keep the select COMPACTION in this patch.
> 

Agreed.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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