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

[Jan Beulich]

>>> On 20.09.12 at 15:49, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
> 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?

Right, but the guest can't really predict this. That may have
significant impact if the performance difference is big enough.

Jan

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