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 15:05
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] SMP and Memory Limits
> 
> 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? :-)

Reasonably so. But I wouldn't bet any more than the smaller
denominations of change that I've currently got in my pocket on it...
;-)

--
Mats
> 
> Gordan
> 
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> 


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