linux-kernel - Re: [PATCH 0/7] Per-bdi writeback flusher threads v20

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Date:	Mon, 21 Sep 2009 18:18:50 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Theodore Tso <tytso@....edu>, Jens Axboe <jens.axboe@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"chris.mason@...cle.com" <chris.mason@...cle.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/7] Per-bdi writeback flusher threads v20

On Mon, Sep 21, 2009 at 06:02:42PM +0800, Jan Kara wrote:
> On Mon 21-09-09 17:53:26, Wu Fengguang wrote:
> > On Mon, Sep 21, 2009 at 01:35:46PM +0800, Wu Fengguang wrote:
> > > > >   Here is how I'd imaging the writeout logic should work:
> > > > > We would have just two lists - b_dirty and b_more_io. Both would be
> > > > > ordered by dirtied_when.
> > > > 
> > > > Andrew has a very good description for the dirty/io/more_io queues:
> > > > 
> > > > http://lkml.org/lkml/2006/2/7/5
> > > > 
> > > > | So the protocol would be:
> > > > |
> > > > | s_io: contains expired and non-expired dirty inodes, with expired ones at
> > > > | the head.  Unexpired ones (at least) are in time order.
> > > > |
> > > > | s_more_io: contains dirty expired inodes which haven't been fully written. 
> > > > | Ordering doesn't matter (unless someone goes and changes
> > > > | dirty_expire_centisecs - but as long as we don't do anything really bad in
> > > > | response to this we'll be OK).
> > > > |
> > > > | s_dirty: contains expired and non-expired dirty inodes.  The non-expired
> > > > | ones are in time-of-dirtying order.
> > > > 
> > > > Since then s_io was changed to hold only _expired_ dirty inodes at the
> > > > beginning of a full scan. It serves as a bounded set of dirty inodes.
> > > > So that when finished a full scan of it, the writeback can go on to
> > > > the next superblock, and old dirty files' writeback won't be delayed
> > > > infinitely by poring in newly dirty files.
> > > > 
> > > > It seems that the boundary could also be provided by some
> > > > older_than_this timestamp. So removal of b_io is possible
> > > > at least on this purpose.
> > > 
> > > Yeah, this is a scratch patch to remove b_io, I see no obvious
> > > difficulties in doing so.
> > 
> > However the removal of b_io is not that good for possible b_dirty
> > optimizations. For example, we could use a tree for b_dirty for more
> > flexible ordering. Or can introduce a b_dirty_atime to hold the inodes
> > dirtied by atime and expire them much lazily:
> > 
> >                        expire > 30m
> >         b_dirty_atime --------------+
> >                                     |
> >                                     +--- b_io ---> writeback
> >                                     |
> >         b_dirty --------------------+
> >                        expire > 30s
>   Well, you can still implement the above without a need for b_io list. The
> kupdate-style writeback can for example check the first inode in both lists
> and process the inode which is expired for a longer time.

OK. Given that rel_atime is default now, such optimization seems less
useful anyway.

Thanks,
Fengguang
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