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Re: [Xen-users] xen dom0 32 bits memory is limited to 16GB?

To: "Marco Strullato" <marco.strullato@xxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] xen dom0 32 bits memory is limited to 16GB?
From: "Jeff Lane" <sundowner225@xxxxxxxxx>
Date: Tue, 29 Apr 2008 09:10:38 -0400
Delivery-date: Tue, 29 Apr 2008 06:11:09 -0700
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On Tue, Apr 29, 2008 at 7:08 AM, Marco Strullato
<marco.strullato@xxxxxxxxx> wrote:
> Hi all,
>  I'm reading that xen dom0 32 bits is limited to 16GB of ram (see
>  http://www.centos.org/modules/newbb/viewtopic.php?topic_id=12690)
>  Is it true? How can I remove this limit?

The ultimate limit should be 64GB using PAE enabled kernels.  I've
done this before, but there are some things you should know...

First, I've found that ultimately, you really should limit dom0 to
about 1-4GB or so.  The main purpose for dom0 is to handle all the
domUs.  I've run into many conditions before where the code that
balloons dom0 memory to free RAM for domUs can fail, causing the
machine to become unstable or to crash, or on the less catastrophic
end, simply cause Xen to stop allocating RAM to domUs.

Second,  I've run 32bit Xen w/ 32bit OSs and I have never seen them
stable at 64GB in any case.  Normally, I can get to about 48GB (I
think that's the support limit for Red Hat Enterprise and SLES).  So
anything more than, say 32GB to be safe should probably be limited to
48-54GB max.  But that's just been my experience with Xen on large

another thing you need to consider, if your system is NUMA capable, is
to enable NUMA (numa=on) to help with performance, especially on
multi-core large memory configurations.


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