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Date: Tue, 6 Feb 2001 13:09:09 -0500 (EST)
From: Ben LaHaise <bcrl@redhat.com>
To: Jens Axboe <axboe@suse.de>
cc: "Stephen C. Tweedie" <sct@redhat.com>,
        Linus Torvalds <torvalds@transmeta.com>,
        Alan Cox <alan@lxorguk.ukuu.org.uk>,
        Manfred Spraul <manfred@colorfullife.com>, Steve Lord <lord@sgi.com>,
        <linux-kernel@vger.kernel.org>,
        <kiobuf-io-devel@lists.sourceforge.net>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait
In-Reply-To: <20010206190018.E580@suse.de>
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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.

		-ben

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