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

[Dave Chinner <david@fromorbit.com>]

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@oracle.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...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com