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Re: [Xen-devel] memory size inside domU less than memory in domU config

On Fri, Jun 17, 2011 at 02:03:16PM +0400, Vasiliy G Tolstov wrote:
> On Fri, 2011-06-17 at 10:51 +0100, Jan Beulich wrote:
> > >>> On 16.06.11 at 20:19, Vasiliy G Tolstov <v.tolstov@xxxxxxxxx> wrote:
> > > Hello. I'm try to use linux-3.0.0-rc1 in domU. 
> > > Domain created with memory=2048 and maxmemory=4096. Inside domU i have 
> > > cat /proc/meminfo  | grep MemTotal
> > > MemTotal:        2016564 kB
> > > 
> > > Why. Kernel boot log and config in attached files. How can i minimize
> > > memory difference with config value and inside domU?
> > 
> > Did you ever look at MemTotal on a physical machine with 2Gb installed?
> > 
> I do not have 2Gb, but have 1Gb
> cat /proc/meminfo 
> MemTotal:        1021320 kB
> this is smaller than 1024, but not to 78Mb.. 

So I tried the same experiment - a physical machine with 'mem=1G' provided
and an PV guest with 'mem=1G'. The same exact kernel in both cases and got:

Machine with 'mem=1G' on the command line.
[    0.000000] Memory: 1022756k/1048576k available (5851k kernel code, 452k 
absent, 25368k reserved, 2881k data, 688k init)

PV guest:
[    0.000000] Memory: 1018888k/1048576k available (5851k kernel code, 448k 
absent, 29240k reserved, 2881k data, 688k init)

So a DomU guest "eats" an extra 4MB compared to running it under baremetal.
(Note, the DomU guest has no balloon space in this example).

The memblock=debug does provide the info of what the "other" reserved regions 
and it looks to be the matter of just going through the numbers to figure out 
it is.

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