linux-kernel - Re: [PATCH 2/7] writeback: switch to per-bdi threads for flushing data

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Message-ID: <20090316233835.GM26138@disturbed>
Date:	Tue, 17 Mar 2009 10:38:35 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	chris.mason@...cle.com, npiggin@...e.de
Subject: Re: [PATCH 2/7] writeback: switch to per-bdi threads for flushing
	data

On Mon, Mar 16, 2009 at 08:33:21AM +0100, Jens Axboe wrote:
> On Mon, Mar 16 2009, Dave Chinner wrote:
> > On Fri, Mar 13, 2009 at 11:54:46AM +0100, Jens Axboe wrote:
> > > On Thu, Mar 12 2009, Andrew Morton wrote:
> > > > On Thu, 12 Mar 2009 15:33:43 +0100 Jens Axboe <jens.axboe@...cle.com> wrote:
> > > > Bear in mind that the XFS guys found that one thread per fs had
> > > > insufficient CPU power to keep up with fast devices.
> > > 
> > > Yes, I definitely want to experiment with > 1 thread per device in the
> > > near future.
> > 
> > The question here is how to do this efficiently. Even if XFS is
> > operating on a single device, it is not optimal just to throw
> > multiple threads at the bdi. Ideally we want a thread per region
> > (allocation group) of the filesystem as each allocation group has
> > it's own inode cache (radix tree) to traverse. These traversals can
> > be done completely in parallel and won't contend either at the
> > traversal level or in the IO hardware....
> > 
> > i.e. what I'd like to see is the ability so any new flushing
> > mechanism to be able to offload responsibility of tracking,
> > traversing and flushing of dirty inodes to the filesystem.
> > Filesystems that don't do such things could use a generic
> > bdi-based implementation.
> > 
> > FWIW, we also want to avoid the current pattern of flushing
> > data, then the inode, then data, then the inode, ....
> > By offloading into the filesystem, this writeback ordering can
> > be done as efficiently as possible for each given filesystem.
> > XFs already has all the hooks to be able to do this
> > effectively....
> > 
> > I know that Christoph was doing some work towards this end;
> > perhaps he can throw his 2c worth in here...
> 
> This is very useful feedback, thanks Dave. So on the filesystem vs bdi
> side, XFS could register a bdi per allocation group.

How do multiple bdis on a single block device interact?

> Then set the proper
> inode->mapping->backing_dev_info from sb->s_op->alloc_inode and
> __mark_inode_dirty() should get the placement right. For private
> traverse and flush, provide some address_space op to override
> generic_sync_bdi_inodes().

Yes, that seems like it would support the sort of internal XFS
structure I've been thinking of.

> It sounds like I should move the bdi flushing bits separate from the bdi
> itself. Embed one in the bdi, but allow outside registration of others.
> Will fit better with the need for more than one flusher per backing
> device.

*nod*

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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