linux-kernel - Re: [RFC] page-writeback: move indoes from one superblock together

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Date:	Thu, 24 Sep 2009 22:09:19 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Mason <chris.mason@...cle.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Jan Kara <jack@...e.cz>
Subject: Re: [RFC] page-writeback: move indoes from one superblock together

On Thu, Sep 24, 2009 at 09:52:17PM +0800, Arjan van de Ven wrote:
> On Thu, 24 Sep 2009 21:46:25 +0800
> Wu Fengguang <fengguang.wu@...el.com> wrote:
> > 
> > Note that dirty_time may not be unique, so need some workaround.  And
> > the resulted rbtree implementation may not be more efficient than
> > several list traversals even for a very large list (as long as
> > superblocks numbers are low).
> > 
> > The good side is, once sb+dirty_time rbtree is implemented, it should
> > be trivial to switch the key to sb+inode_number (also may not be
> > unique), and to do location ordered writeback ;)
> 
> would you want to sort by dirty time, or by inode number?
> (assuming inode number is loosely related to location on disk)

Sort by inode number; dirty time will also be considered when judging
whether the traversed inode is old enough(*) to be eligible for writeback.

(*) this "old enough" criterion has to be much more relaxed, from the
original >30s to >5s. The promise to user would change from

        "dirtied inodes will be started writeback _around_ 30s"

to

        "dirtied inodes will be started writeback _within_ 30s"



The more detailed algorithm would be:

- put inodes to rbtree with key sb+inode_number
- in each per-5s writeback, traverse a range of 1/5 rbtree
- in each traverse, sync inodes that is dirtied more than 5s ago

So the user visible result would be
- on every 5s, roughly a 1/5 disk area will be visited
- for each dirtied inode, it will be synced after 5-30s


Thanks,
Fengguang
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