Re: [Xen-devel] Assigning contiguous memory to a driver domain

[Goswin von Brederlow <goswin-v-b@xxxxxx>]

Rafal Wojtczuk <rafal@xxxxxxxxxxxxxxxxxxxxxx> writes:

> On Wed, Sep 15, 2010 at 01:58:30PM +0200, Goswin von Brederlow wrote:
>> "Jan Beulich" <JBeulich@xxxxxxxxxx> writes:
>> 
>> >> Well, we do have 2 driver domains in Qubes: one is the netvm (NIC
>> >> drivers), and the other one is Dom0 (storage, video, etc). BTW, how is
>> >> it that the drivers in Dom0 never has this problem with allocating
>> >> continues memory (even if we keep xen free memory = 0)?
>> >
>> > Merely because post-boot there would not usually be allocations.
>> 
>> It is also called fragmentation.
>> 
>> dom0 gets its memory at the start and probably all in one big
>> chunk. Other domains have to use the bits and pices that happen to be
>> free when the domain is started.
>
> It does not look right. Jan has already written
> * No, how (dis)contiguous the memory of a domain is doesn't matter
> * here. What matters is whether the domain can *make* the requested
> * memory contiguous, and that depends on how much contiguous
> * memory Xen has at the point of the allocation.

But that is exactly the point. By the time the later domains start the
remaining memory is fragmented and there just might not be a physical
continious chunk of memory there that XEN can give the domain.

The more memory is free the greater the chance that a sufficient chunk
exists. But that is just a probability game. In the worst case you can
have 50% free and no chunk larger than a page.

MfG
        Goswin

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