linux-kernel - Re: [PATCH v6 0/4] fadvise(DONTNEED) support

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Message-ID: <20110222171158.GC31195@random.random>
Date:	Tue, 22 Feb 2011 18:11:58 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Jeffrey Hundstad <jeffrey.hundstad@...u.edu>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Barrett <damentz@...uorix.net>,
	Ben Gamari <bgamari.foss@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mel@....ul.ie>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH v6 0/4] fadvise(DONTNEED) support

On Tue, Feb 22, 2011 at 11:03:51AM -0600, Jeffrey Hundstad wrote:
> On 02/22/2011 07:28 AM, Andrea Arcangeli wrote:
> > Hi Minchan,
> > That's my point, we should check if the "thrashing horribly" is really
> > a "recently" or if it has always happened before with 2.6.18 and previous.
> 
> I would bet that "thrashing" isn't what he meant.  He almost certainly 
> meant "needless I/O" and nothing to do with swapping.  I've seen this 
> similar report before.  This report is almost always gets answered with 
> a "set your swappiness to an appropriate value" type answer.  Or a "your 

Hmm but swappiness can't help if "needless I/O in the morning after
backup" is the problem. So it wouldn't be the right suggestion if
"needless I/O is the problem".

We need a "vmstat 1" during the "trashing horribly"/"needless I/O" in
the morning to be sure.

> userspace app needs to get smarter" type answer.  I think they are 
> trying to do the latter, but need some help from the kernel, and that's 
> what they're trying to do here.

That's fine thing. But I got reports indipendent of this thread from
friends, asking me how to prevent swapping overnight because of
backup, so I connected that report to the above "trashing
horribly"/recently. I had no problems personally but may backup runs
once (not twice to activate/reference pages). I had no time to
investigate this further yet to see if things works ok for me even
with a rsync loop activating and referencing everything.
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