Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait

From: Ben LaHaise (
Date: Tue Feb 06 2001 - 13:09:09 EST

On Tue, 6 Feb 2001, Jens Axboe wrote:

> Stephen already covered this point, the merging is not a problem
> to deal with for read-ahead. The underlying system can easily

I just wanted to make sure that was clear =)

> queue that in nice big chunks. Delayed allocation makes it
> easier to to flush big chunks as well. I seem to recall the xfs people
> having problems with the lack of merging causing a performance hit
> on smaller I/O.

That's where readaround buffers come into play. If we have a fixed number
of readaround buffers that are used when small ios are issued, they should
provide a low overhead means of substantially improving things like find
(which reads many nearby inodes out of order but sequentially). I need to
implement this can get cache hit rates for various workloads. ;-)

> Of course merging doesn't have to happen in ll_rw_blk.
> > As for io completion, can't we just issue seperate requests for the
> > critical data and the readahead? That way for SCSI disks, the important
> > io should be finished while the readahead can continue. Thoughts?
> Priorities?

Definately. I'd like to be able to issue readaheads with a "don't bother
executing if this request unless the cost is low" bit set. It might also
be helpful for heavy multiuser loads (or even a single user with multiple
processes) to ensure progress is made for others.


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