linux-kernel - Re: [PATCH] mm/readahead.c: update the LRU positions of in-core pages, too

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Date:	Wed, 27 Jan 2010 20:21:57 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Chris Frost <frost@...ucla.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steve Dickson <steved@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Xu Chenfeng <xcf@...c.edu.cn>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steve VanDeBogart <vandebo-lkml@...dbox.net>
Subject: Re: [PATCH] mm/readahead.c: update the LRU positions of in-core
	pages, too

Hi Minchan,

On Wed, Jan 27, 2010 at 09:09:40AM +0200, Minchan Kim wrote:
> Hi, Wu.
> 
> I have missed this thread until now.
> Before review, first of all, Thanks for adding to good feature, Chris and Wu.
> I have some questions.
> 
> 2010/1/21 Wu Fengguang <fengguang.wu@...el.com>:
> 
> > Years ago I wrote a similar function, which can be called for both
> > in-kernel-readahead (when it decides not to bring in new pages, but
> > only retain existing pages) and fadvise-readahead (where it want to
> > read new pages as well as retain existing pages).
> 
> Why doesn't it merged into mainline?
> It's private patch or has some problem?

It's part of the early adaptive readahead patchset, which is too
complex to be acceptable to mainline.

> Actually I am worried about this patch.
> That's because it makes shortcut promotion in reclaim exceptionally.
> 
> Of course If readahead is working well, this patch effect also would
> be good. But let's think about it.
> 
> This patch effect happens when inactive file list is small, I think.
> It means it's high memory pressure. so if we move ra pages into
> head of inactive list, other application which require free page urgently
> suffer from latency or are killed.
> 
> If VM don't have this patch, of course ra pages are discarded and
> then I/O performance would be bad. but as I mentioned, it's time
> high memory pressure. so I/O performance low makes system
> natural throttling. It can help out of  system memory pressure.
> 
> In summary I think it's good about viewpoint of I/O but I am not sure
> it's good about viewpoint of system.
> 
> I will review this patch after my concern is solved. :)

That's legitimate concern. I'm now including this patch in a bigger
patchset to do adaptive (wrt. thrashing safety) readahead, which will
automatically back off readahead size when thrashing happened. I hope
that would address your concern.

Thanks,
Fengguang
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