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

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Date:	Wed, 19 Sep 2012 13:03:52 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Oliver Chick" <oliver.chick@...rix.com>
Cc:	<xen-devel@...ts.xen.org>, <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] Persistent grant maps for xen blk
 drivers

>>> On 19.09.12 at 12:51, Oliver Chick <oliver.chick@...rix.com> 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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