linux-kernel - Re: [PATCH 0/7] sg_ring: a ring of scatterlist arrays

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Message-Id: <1199547072.3399.15.camel@localhost.localdomain>
Date:	Sat, 05 Jan 2008 09:31:12 -0600
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Jens Axboe <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org
Subject: Re: [PATCH 0/7] sg_ring: a ring of scatterlist arrays

On Wed, 2007-12-19 at 17:31 +1100, Rusty Russell wrote:
> This patch series is the start of my attempt to simplify and make explicit
> the chained scatterlist logic.
> 
> It's not complete: my SATA box boots and seems happy, but all the other
> users of SCSI need to be updated and checked.  But I've gotten far enough
> to believe it's worth persuing.

Sorry for the delay in looking at this, I was busy with Holidays and
things.

When I compare sg_ring with the current sg_chain (and later sg_table)
implementations, I'm actually struck by how similar they are.

The other thing I note is that the problem you're claiming to solve with
sg_ring (the ability to add extra scatterlists to the front or the back
of an existing one) is already solved with sg_chain, so the only real
advantage of sg_ring was that it contains explicit counts, which
sg_table (in -mm) also introduces.

The other differences are that sg_ring only allows adding at the front
or back of an existing sg_ring, it doesn't allow splicing at any point
like sg_chain does, so I'd say it's less functional (not that I actually
want anyone ever to do this, of course ...)

The final point is that sg_ring requires a two level traversal:  ring
list then scatterlist, whereas sg_chain only requires a single level
traversal.  I grant that we can abstract out the traversal into
something that would make users think they're only doing a single level,
but I don't see what the extra level really buys us.

The above analysis seems to suggest that sg_chain is simpler and has
more functionality than sg_ring, unless I've missed anything?

The only thing missing from sg_chain perhaps is an accessor function
that does the splicing, which I can easily construct if you want to try
it out in virtio.

James

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