linux-kernel - Re: [PATCH 2/3] mm: Implement writeback livelock avoidance using page tagging

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Message-ID: <20100215154751.GG3434@quack.suse.cz>
Date:	Mon, 15 Feb 2010 16:47:51 +0100
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	npiggin@...e.de, fengguang.wu@...el.com
Subject: Re: [PATCH 2/3] mm: Implement writeback livelock avoidance using
 page tagging

On Fri 12-02-10 11:39:55, Andrew Morton wrote:
> On Fri, 12 Feb 2010 00:06:23 +0100
> Jan Kara <jack@...e.cz> wrote:
> 
> > The idea is simple: Tag all pages that should be written back
> > with a special tag (TOWRITE) in the radix tree. This can be done
> > rather quickly and thus livelocks should not happen in practice.
> > Then we start doing the hard work of locking pages and sending
> > them to disk only for those pages that have TOWRITE tag set.
> 
> Adding a second pass across all the pages sounds expensive?
  Strictly speaking it's just through the radix tree and only through
branches with DIRTY_TAG set. But yes, there is some additional CPU cost.
I just thought that given the total cost of submitting a page it is
an acceptable increase and the simplification is worth it.
  Would some numbers make you happier? Any suggestion for measurements?
Because I think that even for writes to tmpfs the change will be lost
in the noise...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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