[Libvir] Request for minimum memory level of dom0 (xen)

[Daniel P. Berrange]

On Wed, May 02, 2007 at 02:30:53PM +0200, Jan Michael wrote:
> Hi LibVirt-Mailreaders,
> 
> Is it possible to provide information about the minimum memory level of
> domain 0 (xen)?
> The value is located in the xen deamon config file as "dom0-min-mem".
> 
> I would like to calculate on the basis of this value if there is enough
> memory left to create additional domains.

I've been thinking about this & I'm not sure that dom0-min-mem is really
something we want to expose, particularly for the scenario you describe.
dom0-min-mem is a completely artificial threshold set by Xen that does
not really provide a very reliable solution to determining whether there
is enough memory to start new domains. dom0-min0-mem may be set to 128 MB,
but if your Dom0 is using 500 MB currently, then you can't really say that
you can safely balloon it down to 128MB - it'll end up with swap-death.

In the QEMU / KVM case, there is no concept of dom0-min-mem at all, and
you not only have to take in account physical memory, and memory of the
host, but also memory which may be being used by other apps in the
host.

If an app needs to decide whether there is enough memory on a host to
safely create a new guest, then it really needs to be considering the
actual usage within the host OS - eg, the finer breakdown of figures
from /proc/meminfo for example at the very least. For more advanced
uses one would also want agents inside every guest reporting whether
they're actually using their complete allocation of RAM too. And so on,
expanding to look at all sorts of aspects of the system.

So in this context Dom0-min-mem is not really a generally applicable
concept in virtualization, and it is also not particularly useful
data either IMHO.

Regards,
Dan.
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