[Xen-users] XCP Memory static/dynamic and overcommit

[David Erickson <halcyon1981@xxxxxxxxx>]

Hi all,
So I have been playing around with XCP and the static/dynamic memory
parameters.  I have a few behavioral questions I would like to pin
down:

-Is the static-max quantity of free memory on the host always required
before the guest vm can be started? I assume so since you don't know
a-priori if the guest you are booting supports Xen or not.  But if
this is true, what is the use of static-min?  When I boot a guest does
it just determine the highest memory it can take in the range of
static-min to static-max, given any ability to shrink other guests
that have Xen-enabled kernels?

-For guests running xen-enabled kernels, wouldn't it actually be
better if dynamic-max could be higher than static-max?  IE you could
imagine that you have a lot of VMs running on one host, to start new
ones you need to have them boot with a small amount of physical memory
(say 256MB), but if any one of them is under memory pressure you would
like it to be able to grow up to some cap, say 1024MB or some such,
pending free memory being available to pull from other guests, or just
plain free on the host.

-I have a host with 4GB of memory, I configured 3 debian lenny guests
all running the xen-enabled kernel, they were set to have static max
of 3GB, static min of 256MB, dynamic-max of 512MB, dynamic-min of
256MB. I logged in to one of them and put significant memory pressure
on it, hoping I could get guest's memory to grow while the others were
idle.  However my experience was the guest's would set their memory
directly at whatever dynamic-max is set to.  Is there any way for the
guests to adjust their memory footprint on the fly based on their
memory pressure?  IE what I'd really like is:

--boot-memory: the quantity of memory used to boot the guest, similar
to static-max
--dynamic-max: the largest quantity of memory the guest could
potentially grow to, this could be greater than boot-memory

And then through a combination of ballooning, etc, for kernel
supported guests you could keep the actual dynamic memory as low as
possible (without damaging performance), but allow other guests that
need to temporarily grow/shrink to do so.  This would all need some
sort of fairness policy etc.  Is anything like this currently enabled
in XCP? And if not, what components exist, or would be needed for
something like this?

Thanks,
David

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