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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.09.12 at 13:30, Oliver Chick <oliver.chick@xxxxxxxxxx> wrote:
> The memory overhead, and fallback mode points are related:
> -Firstly, it turns out that the overhead is actually 2.75MB, not 11MB
> per device. I made a mistake (pointed out by Jan) as the maximum number
> of requests that can fit into a single-page ring is 64, not 256.
> -Clearly, this still scales linearly. So the problem of memory footprint
> will occur with more VMs, or block devices.
> -Whilst 2.75MB per device is probably acceptable (?), if we start using
> multipage rings, then we might not want to have
> BLKIF_MAX_PERS_REQUESTS_PER_DEVICE==__RING_SIZE, as this will cause the
> memory overhead to increase. This is why I have implemented the
> 'fallback' mode. With a multipage ring, it seems reasonable to want the
> first $x$ grefs seen by blkback to be treated as persistent, and any
> later ones to be non-persistent. Does that seem sensible?

>From a resource usage pov, perhaps. But this will get the guest
entirely unpredictable performance. Plus I don't think 11Mb of
_virtual_ space is unacceptable overhead in a 64-bit kernel. If
you really want/need this in a 32-bit one, then perhaps some
other alternatives would be needed (and persistent grants may
not be the right approach there in the first place).

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel