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Re: [Xen-users] SMP and Memory Limits

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] SMP and Memory Limits
From: Gordan Bobic <gordan@xxxxxxxxxx>
Date: Tue, 20 Sep 2005 15:05:10 +0100
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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.
So, you are reasonably confident that when applied to a 64GB/16-way 
machine it isn't going to fall over flat on it's face? :-)
Gordan

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