Re: [Xen-users] CentOS6 & Ubuntu12 PV domU Memory

[George Shuklin <george.shuklin@xxxxxxxxx>]

On 05.02.2013 01:58, Rushikesh Jadhav wrote:

On Tue, Feb 5, 2013 at 2:05 AM, George Shuklin <george.shuklin@xxxxxxxxx> 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, Âand 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 ? )

When you start domain it reserve memory to page tables and other stuff according to static-max. When you going below static max, it actually the xen-balloon gives back to xen some of pages from domU. But all internal data structures in kernel are designed to sustain up to static-max value.

But yes, it is very huge amount at my opinion (I expect less). I still in debug reseach, but anyway, this is not xapi/xcp issue, this is pure linux stuff.

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