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Re: [Xen-devel] hypervisor memory usage

What bothers me with this issue is that memory is 'lost' just after boot, without any of domUs running.

Also, 8kb per 1MB would add up to 64MB per 8GB of ram, then I add 1 more MB for dom0 (when counted as guest).

Global hypervisor memory, that is question now, can it use 384MB straight after boot? - memory loss is detected at very first lines of dom0 dmesg.

Is there a way to show more xen debugging before dom0 kicks in?

Pasi Kärkkäinen wrote:
On Wed, Oct 28, 2009 at 10:29:37AM +0100, Vladimir Zidar wrote:

** 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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