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

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Tue, Jan 12, 2010 at 04:57:42PM -0800, David Erickson wrote:
> 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:
>

Hello,

Did you read XCP and/or XenServer docs? 
I'm not sure if those will answer the questions below, but at least
worth checking out.

-- Pasi


> -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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