Re: [PATCH] Domain creation 4MB memory allocation

[Michal Novotny <minovotn@redhat.com>]

Jan Beulich wrote:
>>>> 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
>
>   
Well, in fact setting it to 4MiB is no guarantee at all and this 
information was done only by testing. Anyway the test case written above 
is the problem I run into and this solved this issue. 2MiB was surely 
not enough in my case and therefore I needed to patch it. Any other 
ideas how to make it running fine? I've tested it on some other 
configurations/platforms and it was working with no problems so 2MiB 
were really not enough.

Michal