Xen users - Dec 2010 - 32bit paravirt guests on large memory machines

Hi Xen-Users

My question relates to this comment in the xend config file on my SLES 11 
SP1 installation

--> snip

# 32-bit paravirtual domains can only consume physical
# memory below 168GB. On systems with memory beyond that address,
# they''ll be confined to memory below 128GB.
# Using total_available_memory (in GB) to specify the amount of memory reserved
# in the memory pool exclusively for 32-bit paravirtual domains.
# Additionally you should use dom0_mem = <-Value> as a parameter in
# xen kernel to reserve the memory for 32-bit paravirtual domains, default
# is "0" (0GB).
(total_available_memory 0)

--> snip

I have a system with 256 GB of RAM (and many cores). Our intention had 
been to run a large number of 32 bit paravirtual guests, with each guest 
having 4 or 8 cores and 16 GB RAM assigned, on that system.  The 
restrictions indicated above pose a problem.

1) Can someone clarify if this restriction is a Xen restriction, which is 
possible or likely to be lifted/relaxed at some future date?  I note that 
Xen on SLES is based on xen-4.0.0. Or is it a (hardware/addressing/...) 
limitation that simply cannot be "fixed"?

2) Have I interpreted correctly, in that with Xen 4 I will not be able to 
use any memory above 128GB for any 32 bit para-virtualised guests, 
independent of the kernel/config/release of the guest itself? Experiment 
has led me to suppose my interpretation is correct, but I feel worth 
double checking?

3) Is the restriction also independent of the Xen hypervisor used, e.g. it 
does not matter if the hypervisor is 32 bit or 64 bit?

Thanks,
Kevin

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Hi

No-one any comment/experience/knowledge on the issue below? Should I be 
asking on xen-devel?

Thanks,
Kevin

> Hi Xen-Users
>
> My question relates to this comment in the xend config file on my SLES 11
SP1
> installation
>
> --> snip
>
> # 32-bit paravirtual domains can only consume physical
> # memory below 168GB. On systems with memory beyond that address,
> # they''ll be confined to memory below 128GB.
> # Using total_available_memory (in GB) to specify the amount of memory 
> reserved
> # in the memory pool exclusively for 32-bit paravirtual domains.
> # Additionally you should use dom0_mem = <-Value> as a parameter in
> # xen kernel to reserve the memory for 32-bit paravirtual domains, default
> # is "0" (0GB).
> (total_available_memory 0)
>
> --> snip
>
> I have a system with 256 GB of RAM (and many cores). Our intention had been
> to run a large number of 32 bit paravirtual guests, with each guest having
4
> or 8 cores and 16 GB RAM assigned, on that system.  The restrictions 
> indicated above pose a problem.
>
> 1) Can someone clarify if this restriction is a Xen restriction, which is 
> possible or likely to be lifted/relaxed at some future date?  I note that
Xen
> on SLES is based on xen-4.0.0. Or is it a (hardware/addressing/...) 
> limitation that simply cannot be "fixed"?
>
> 2) Have I interpreted correctly, in that with Xen 4 I will not be able to
use
> any memory above 128GB for any 32 bit para-virtualised guests, independent
of
> the kernel/config/release of the guest itself? Experiment has led me to 
> suppose my interpretation is correct, but I feel worth double checking?
>
> 3) Is the restriction also independent of the Xen hypervisor used, e.g. it 
> does not matter if the hypervisor is 32 bit or 64 bit?
>
> Thanks,
> Kevin
>
> _______________________________________________
> Xen-users mailing list
> Xen-users@lists.xensource.com
> http://lists.xensource.com/xen-users
>
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