Re: HVM and free_memory query

[Keir Fraser <keir.fraser@eu.citrix.com>]

On 09/07/2009 20:28, "Apoorva Kulkarni" <apoorva@3tera.com> wrote:

> #xm list
> Name                                        ID   Mem VCPUs      State
> Time(s)
> Domain-0                                     0   512     1     r-----  25251.4
> controller                                   1   512     1     -b----  39948.6
> vm.windows03                                44 30720     1     r-----     22.0
> 
> As you can see, the free_memory shown is 337M which is a good 241M less than
> what it should have been (31298M - 30720M = 578M) after the Windows VM was
> created. My question here is where/why does xen use that extra 241M. Any
> pointers much appreciated.

It's used for shadow pagetables and other memory book-keeping structures.
This reservation is sized as a fraction of the domain's configured memory
allocation -- the fraction is just under 0.8%. And 0.8% of 30720MB is 245MB,
so that works out just about right.

This is one of those cases where a small fraction of a very big number is
still a big number. ;-)

 -- Keir