Re: [Xen-devel] [PATCH] xen: blkif.h: document linux xen-block multi-page ring implementation

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx]
> Sent: 15 May 2015 16:23
> To: Bob Liu; msw@xxxxxxxxxx; Paul Durrant; Keir (Xen.org)
> Cc: xen-devel@xxxxxxxxxxxxx; David Vrabel; justing@xxxxxxxxxxxxxxxx; Roger
> Pau Monne
> Subject: Re: [PATCH] xen: blkif.h: document linux xen-block multi-page ring
> implementation
> 
> On Tue, May 12, 2015 at 06:58:49PM +0800, Bob Liu wrote:
> > After commit 1b1586eeeb8c ("xenbus_client: Extend interface to
> > support multi-page ring"), Linux xenbus driver can support multi-page ring.
> >
> > Based on this interface, we got some impressive improvements by using
> multi-page
> > ring in xen-block driver. If using 64 pages as the ring, the IOPS increased
> > about 15 times for the throughput testing.
> >
> > The Linux implementation reuses two 'DEPRECATED' nodes('max-ring-
> pages' and
> > 'num-ring-pages), so that nothing would be broken.
> > Also removed the power of 2 limit and updated the default/max value
> accordingly.
> 
> Doing an git annotate shows:
> 
> commit 829f2a9c6dfae13faee4e24b953441113338e0f2
> Author: Justin T. Gibbs <justing@xxxxxxxxxxxxxxxx>
> Date:   Thu Feb 23 10:02:30 2012 +0000
> 
>     blkif.h: Document the Red Hat and Citrix blkif multi-page ring extensions
> 
>     No functional changes.
> 
> 
> Which says:
> 
>  * (1) Multi-page ring buffer scheme first developed in the Citrix XenServer
> + *     PV drivers.
> + * (2) Multi-page ring buffer scheme first used in some RedHat distributions
> + *     including a distribution deployed on certain nodes of the Amazon
> + *     EC2 cluster.
> + * (3) Support for multi-page ring buffers was implemented independently,
> + *     in slightly different forms, by both Citrix and RedHat/Amazon.
> 
> 
> And I double checked the linux-2.6.18.hg tree and I see no mention of
> multi-page. I wonder where such code exists?
> 
> CC-ing Matt, perhaps he can help.
> 
> Cc-ing Paul, perhaps he can help with the Citrix part.
> 

The only code that has ever been deployed by Citrix, to my knowledge, is the 
Windows multi-page frontend code which uses the 'page-order' variants of the 
xenstore keys. I don't believe Citrix has ever shipped capable backends in 
XenServer, but it's possible Amazon implemented such a thing since they use 
Citrix drivers in their Windows guests.

  Paul

> Thank you!
> 
> P.S.
> Also added Keir on the 'To' as scripts/get_maintainers.pl says he should be
> on it.
> 
> >
> > Signed-off-by: Bob Liu <bob.liu@xxxxxxxxxx>
> > ---
> >  xen/include/public/io/blkif.h |   12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/xen/include/public/io/blkif.h b/xen/include/public/io/blkif.h
> > index 6baf7fb..0e34ae6 100644
> > --- a/xen/include/public/io/blkif.h
> > +++ b/xen/include/public/io/blkif.h
> > @@ -189,11 +189,11 @@
> >   *
> >   * max-ring-pages
> >   *      Values:         <uint32_t>
> > - *      Default Value:  1
> > - *      Notes:          DEPRECATED, 2, 3
> > + *      Default Value:  32
> > + *      Notes:          2, 3
> >   *
> >   *      The maximum supported size of the request ring buffer in units of
> > - *      machine pages.  The value must be a power of 2.
> > + *      machine pages.
> >   *
> >   *------------------------- Backend Device Properties 
> > -------------------------
> >   *
> > @@ -302,11 +302,11 @@
> >   * num-ring-pages
> >   *      Values:         <uint32_t>
> >   *      Default Value:  1
> > - *      Maximum Value:  MAX(max-ring-pages,(0x1 << max-ring-page-
> order))
> > - *      Notes:          DEPRECATED, 2, 3
> > + *      Maximum Value:  max-ring-pages
> > + *      Notes:          2, 3
> >   *
> >   *      The size of the frontend allocated request ring buffer in units of
> > - *      machine pages.  The value must be a power of 2.
> > + *      machine pages.
> >   *
> >   * feature-persistent
> >   *      Values:         0/1 (boolean)
> > --
> > 1.7.10.4
> >

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