Re: [PATCH] Domain creation 4MB memory allocation

["Jan Beulich" <jbeulich@novell.com>]

>>> Michal Novotny <minovotn@redhat.com> 01.04.09 09:49 >>>
>Hi,
>I've found some problems when running 2 guests with and then trying to 
>install a new machine (no matter whether using virt-manager or 
>virt-install). I am attaching a patch that fixes this problem.
>
>I was working on 8G box and I have done those things:
> 1. xm create PVguest maxmem=2500 memory=2500
> 2. xm create FVguest maxmem=2500 memory=2500
> 3. virt-install -l http://path/to/install/ -r 2500 -v --nodisks -n 
>someGuestName
>
>Before applying this patch it returned "Cannot allocate memory" error 
>but after applying my patch it created a new domain and started 
>installation well but it's reproducible sometimes and not everytime.

This is just curing (actually not really curing, just reducing the likelihood of
its occurrence) a symptom, rather than the root cause: What if 4Mb turn
out to be insufficient for someone else on some other machine? In order
to guarantee hvm guest creation to be possible (at least in the case
where shadow paging is to be used - not sure about NPT/EPT's
requirements), there has to be a way to guarantee some small amount
of contiguous memory being available in Xen. Ballooning cannot guarantee
this, and after a longer period of up-time with many guests starting and
dying even using dom0_mem= on the Xen command line doesn't help due
to fragmentation being unavoidable.

Jan