RE: [Xen-users] SMP and Memory Limits

["Petersson, Mats" <mats.petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Gordan Bobic
> Sent: 20 September 2005 14:27
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] SMP and Memory Limits
> 
> Petersson, Mats wrote:
> >>-----Original Message-----
> >>From: Gordan Bobic [mailto:gordan@xxxxxxxxxx]
> >>Sent: 20 September 2005 12:34
> >>To: Petersson, Mats
> >>Subject: Re: [Xen-users] SMP and Memory Limits
> >>
> >>
> >>>>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. 

--
Mats
> 
> Gordan
> 
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> 


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