Re: [Xen-devel] Why are I/O rings bidirectional?

["Mark A. Williamson" <mark.williamson@xxxxxxxxxxxx>]

> I think the intended question was "why have both requests and responses
> in the same ring rather than have two rings, one for requests and one
> for responses?"

Ah, right.  Sorry.

Well I don't think there is any huge advantage to putting requests and 
responses into the same ring.

The advantages I can see are:
* It's a neat sort of way of doing things
* Provides backpressure against the requester (in that it has to keep removing 
responses in order to keep queuing requests).

You could get the same performance using a two ring setup like you describe, 
so to a certain extent it's just personal taste.

Cheers,
Mark

> On Mon, 2005-02-07 at 23:00 +0000, Mark Williamson wrote:
> > > Hello. I am reading Xen and the Art of Virtualization. I am curious as
> > > to why the async I/O ring buffers contain both requests and responses
> > > in the same ring.
> >
> > Requests contain details of the IO to be done.  Responses notify the
> > domain when the IO is complete and whether it was successful or not.
> >
> > Requests can complete out of order, so it's necessary to have response
> > messages following back to the domain to tell it when each request is
> > done. If the requests were always dealt with in order it would not be
> > necessary to have responses flowing back, so the ring could be
> > unidirectional.
> >
> > The Safe Hardware Access paper has more details about the IO rings work.
> >
> > HTH,
> > Mark
> >
> >
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