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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Sep 20, 2012 at 12:48:41PM +0100, Jan Beulich wrote:
> >>> 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

Wouldn't it fall back to the older performance?
> _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
> 

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