Petersson, Mats wrote:

What are the limits on how many CPUs and how much memory

Xen supports?

I am interested in this for both the host (0) kernel and

the client

(U) kernel.
I am looking at getting some 16-way (8-way dual-core)

Opteron systems

with about 64 GB of RAM for prototyping, so I would like to

make sure

that Xen can use up all of the resources of such a machine.

In Xen-unstable (to become 3.0) I believe there is no

software limits

for CPU count or memory amount, only whatever limits the hardware dictates (i.e. 40 bits of hardware address, 48 bits

available in virtual

space). If there are any other limitations, it's probably fair to consider it a bug, and report such failings on the

Xen-Devel mailing

list.
Obviously, 2.0.x, only supporting 32-bit in non-PAE mode

would not be

able to use more than about 3.5GB of RAM.

Is that 3.5 GB per dom-U/dom-0, or the total between dom-0 and all dom-Us put together?

That would be 3.5 GB in total, since the only way to access

more than

this amount of memory would involve using address extended

page table,

which isn't supported by Xen 2.0.x. 64-bit x86 actually uses the exisiting PAE, but with an added page-table level so that a bigger than 36-bit address can be supported.

How stable is the "unstable/64-bit" version of Xen? Is it usable?

I should think that it's usable as long as you don't expect your system
to be a "production system with 100% 24/7 availability" [or somewhere
where you'd have 100 angry users to answer to if the system goes down
for more than half a minute]. Unstable should really be renamed into
"testing" by now, as that's really what it is, but someone in XenSource
decided that a rename of the directory was too much work... Or some
such... The biggest problem would probably be obscure hardware or things
that are rarely used in Xen, which I'm sure that there are some

outstanding bugs and new ones to crop up in the near future.

Gordan
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