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Re: [Xen-users] Vanishing memory? Whats going on?

To: bleomycin <diabolical@xxxxxxxxx>
Subject: Re: [Xen-users] Vanishing memory? Whats going on?
From: John Haxby <john.haxby@xxxxxxxxxx>
Date: Fri, 04 Jul 2008 10:45:49 +0100
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bleomycin wrote:
Hello everyone, i just installed xen on standard debian etch server. 4gb ram,
Q6600 machine. I am running two hvm guests, one windows 2k3, the other
debian etch, both with 512mb ram allocated. What i dont understand is that
when i start a guest OS the amount of total ram known to the host machine
seems to vanish? Free -m and top both report that with both guests running i
only have 2.9gb of total system memory. If i attempt to start a third guest,
hvm xubuntu, with 512mb of ram allocated to it i get the error "Error: (12,
'Cannot allocate memory')" Please forgive me for such a trivial question,
but i have searched these forums and google to the best of my ability and i
keep coming up blank :(
What you're looking at with "free -m" is, I would imagine, the memory available to dom0 and that would make sense as the hypervisor takes a slice and then each domU takes 0.5GB leaving you with just shy of 3GB.

If you can't create a third domU then its because the balloon driver in dom0 can't reclaim enough memory to start the new domain: generally speaking the balloon driver will "acquire" memory from the cache.

You could try setting dom0_mem=1024M (for example) on the kernel command line in grub (there's a kernel line and a couple of module lines normally). That will leave you with just short of 3GB for guests.

Oh, and if you have the choice then go for pv guests rather than hvm because having a kernel that knows its in a virtual environment is generally an advantage.


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