Re: [Xen-devel] [PATCH] Persistent grant maps for xen blk drivers

[Oliver Chick <oliver.chick@xxxxxxxxxx>]

Ah yes, you're right - rings can accomodate 64 requests. ijc, and myself
got muddled. It also explains the comment in my summary that we only use
the first 60-ish persistent grants!

With multipage rings, this will probably become a function, rather than
a constant. Does that sound sensible?

On Wed, 2012-09-19 at 13:03 +0100, Jan Beulich wrote:
> >>> On 19.09.12 at 12:51, Oliver Chick <oliver.chick@xxxxxxxxxx> wrote:
> > +/*
> > + * Maximum number of persistent grants that can be mapped by Dom0 for each
> > + * interface. This is set to be the size of the ring, as this is a limit on
> > + * the number of requests that can be inflight at any one time. 256 imposes
> > + * an overhead of 11MB of mapped kernel space per interface.
> > + */
> > +#define BLKIF_MAX_PERS_REQUESTS_PER_DEV 256
> 
> How can this be a fixed number, especially in the context of ring
> size extensions? Iirc, single page rings can accommodate 64
> requests, so I'd guess the number above was observed with a
> 4-page (order 2) ring...
> 
> Jan
> 



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