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Re: [Xen-devel] hypervisor memory usage

On Wed, Oct 28, 2009 at 10:29:37AM +0100, Vladimir Zidar wrote:
> Hi,
>  ** I sent this mail originally to xen-users, but after seeing other 
> subjects posted on that list I felt that it was kind of wrong place to 
> ask **
> I'm wondering why hypervisor itself uses so much memory? Is it normal?
> I have several servers with 8GB of ram, all of them running x86_64
> centos 5.4 (latest) and when running under xen, dom0 has
> [root@palmae ~]# head -4 /proc/meminfo
> MemTotal:      7661568 kB
> MemFree:       6550044 kB
> Buffers:         37400 kB
> Cached:         303480 kB
> Which is  440MB less than what I get without xen.
> It comes down to what amount of memory kernel gets either from Xen or
> from BIOS. Please note the differences:

First of all you should limit the amount of memory visible to dom0 by
specifying dom0_mem=512M (or so) parameter for xen.gz in grub.conf.

After that reboot and then you can check the Xen hypervisor free memory with 
"xm info" and list the guest/domain mem usage with "xm list".

Xen has some memory overhead (just like every virtualization solution).

I think the formula for Xen memory overhead is: 8kB per 1MB of guest
memory, plus 1MB per guest virtual cpu. 

I think this also applies to dom0, since basicly it is just a guest 
(with some more privileges).

And in addition of course there is the "global" hypervisor memory
usage.. not sure if those will add up to 440 MB though.

-- Pasi

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