Xen users - Feb 2013 - CentOS6 & Ubuntu12 PV domU Memory

Hi Everyone,

I''m not sure if I''m the only one facing it or is it a known
issue.
CentOS6 and Ubuntu12.04 domU report less memory than they actually have.

Example, a CentOS6 VM with 512MB shows only 484 as total in free -m.
A Ubuntu domU shows 284M as total if the static max is 8GB and target
memory is 512MB.

Both domUs are 64bit.

CentOS5 reports the exact same memory as its target.
I would be thankful if someone can share a reason or their domU # free -m

Regards,
Rushikesh


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On 04.02.2013 22:59, Rushikesh Jadhav wrote:
> Hi Everyone,
>
> I''m not sure if I''m the only one facing it or is it a
known issue.
> CentOS6 and Ubuntu12.04 domU report less memory than they actually have.
>
> Example, a CentOS6 VM with 512MB shows only 484 as total in free -m.
> A Ubuntu domU shows 284M as total if the static max is 8GB and target 
> memory is 512MB.
>
> Both domUs are 64bit.
>
> CentOS5 reports the exact same memory as its target.
> I would be thankful if someone can share a reason or their domU # free -m
>
User-friendly answer: linux specific. Not XCP issue.

Technical stuff: pv-ops version of linux kernel marks some used areas as 
reserved (you can see this in dmesg output in Memory: line). AFAIK (I''m
not sure) -xen version of linux mark those areas as
''available'', but
''busy''. PV-ops kernel simply mark it as
''reserved''.

You can compare ''free'' memory for -xen and pv-ops kernels - it
same. But
for -xen kernel TotalMem higher and Used is higher too,  and for pv-ops 
TotalMem is lower, but used is lower too.

On Tue, Feb 5, 2013 at 2:05 AM, George Shuklin
<george.shuklin@gmail.com>wrote:
> On 04.02.2013 22:59, Rushikesh Jadhav wrote:
>
>> Hi Everyone,
>>
>> I''m not sure if I''m the only one facing it or is it a
known issue.
>> CentOS6 and Ubuntu12.04 domU report less memory than they actually
have.
>>
>> Example, a CentOS6 VM with 512MB shows only 484 as total in free -m.
>> A Ubuntu domU shows 284M as total if the static max is 8GB and target
>> memory is 512MB.
>>
>> Both domUs are 64bit.
>>
>> CentOS5 reports the exact same memory as its target.
>> I would be thankful if someone can share a reason or their domU # free
-m
>>
>>
> User-friendly answer: linux specific. Not XCP issue.
>
> Technical stuff: pv-ops version of linux kernel marks some used areas as
> reserved (you can see this in dmesg output in Memory: line). AFAIK
(I''m not
> sure) -xen version of linux mark those areas as
''available'', but ''busy''.
> PV-ops kernel simply mark it as ''reserved''.
>
> You can compare ''free'' memory for -xen and pv-ops kernels
- it same. But
> for -xen kernel TotalMem higher and Used is higher too,  and for pv-ops
> TotalMem is lower, but used is lower too.
>
>
Thanks George,

I can understand that *some* memory area to be reserved but I dont
understand why does it change with the "static-max" parameter of VM.
Total memory inside the domU is inversely proportional to the difference of
"static-max" and target memory.

In case of ubuntu VM, if static-max=8GB and target=1GB then free -m would
show something near 600-700MB
Whereas, if static-max=2GB and target=1GB then free -m would show near
850MB and if static-max=target=1GB then free -m would show near 950MB.

The difference in available domU memory is quite a big figure with more
than 100MB difference. I''m not sure but its doubtful that xen would
need
such huge reserved area. ( for ballooning ? )




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