Xen devel - Apr 2009 - [PATCH] Domain creation 4MB memory allocation

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.

Best regards,
Michal

Signed-off-by: Michal Novotny <minovotn@redhat.com>


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>>> 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


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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

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