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Re: [Xen-users] Home Xen hypervisor for master's project

To: xen-users <Xen-users@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] Home Xen hypervisor for master's project
From: Joseph Commisso <commissoje@xxxxxxxxx>
Date: Sun, 20 Feb 2011 09:38:38 -0500
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I get the "Daily Digest" version of this forum and I see no way to
reply to messages, so I am generating a new message and copying the
subject to the list, but I do not think it will show up in sequence to
the original messages, like it is supposed to. Oh well..

In reply to Marek:

You wrote:
"This is where the difference between Intel and AMD is the biggest. Intel
seems to be disabling CPU features at random and this will lead to
problems with virtualization.
Assuming you are interested in going with at least a quad core. Three
cores may also be enough, but two would likely be too few if you want to
run several VMs at once. This immediately limits your options to only some
I assume you won't be getting a Sandy Bridge i5 due to the SATA 2
controller issue in the chipset, so the only i5 is Lynnfield (750 and
760), but neither has support for VT-d. All Lynnfield Core i7 CPUs support
VT-d apart from 875K (just so you know, I don't think you were interested
in that one, anyway). Support for VT-d with 900 series depends on the
motherboard and its BIOS.

Conversely, AMD enables everything on its entire CPU range. If it's based
on K10.5, it will have support for AMD-V, memory ECC, and with the right
motherboard (currently limited to 890FX-based), IOMMU. Your choice, but
for virtualization, especially on a budget, nothing beats AMD."

I was thinking about the i5-4500, which has VT-d, but then after
talking to a coworker, I started to consider AMD.
I did not want to wait for the Sandy Bridge fix, but I was still
trying to decide, but that was where I was at a week ago.
I then looked at the Phenom II product line and was trying to decide
on their latest 1100, but did not know about how Xen uses the number
of cores, which prompted this message thread.
Ryan has indicated that Xen will use as many cores as there are, so
based on that, and on his decision to go with the Xeon, I thought that
the 8 core magny cours Opteron would make things real nice for this
After this project is complete, I could decide whether to continue
using the server or sell it off.

You wrote:
> I know that server grade CPUs are constructed totally different than
> general desktop CPUs, but I do not know why or how that would effect
> my processing environment and my experience on this system.
Not totally different. If anything, some features may have been fused off,
but otherwise the dies are exactly the same. Since Intel is using LGA
across the board, they have the same mounting as well (AMD uses PGA
sockets, while all current Opterons use LGA).
One feature missing from consumer version of Intel CPUs is lack of support
for ECC on any of the buses (I'm not sure about cache ECC). This is
important, because if you want to run a server, you'll want to have ECC
protection on pretty much everything, so if you want to go with Intel, get
a Xeon (even i7-900 series don't support ECC memory or QPI). AMD, on the
other hand, does support ECC on both HyperTransport (by design) and on the
memory controller with all current consumer CPUs.
Fundamentally, however, there are no differences between the enterprise
and the consumer versions of a given CPU. If any given software demands a
'server-grade CPU', rather than server-specific features, it's usually
bogus, since, e.g., an Athlon with ECC memory is server grade, while an
Opteron with no-ECC memory is not, regardless of the CPU brand.

I did not know that about the Phenom and i7 as NOT using ECC, but that
is good to know.
At the time, I was going to try and decide on which memory to use,
after I finalized my CPU/motherboard choice.
That is ok, since I am working now on choosing between the 4100 or
6100 series of Opteron.
I need to find out if the 4100 can do the IOMMU as good as the 6100.
I have the data sheets on each and I am going to review them now.
I will then see if I can afford all this with the ECC RAM and all.

You wrote:
Now, all the above assumes that you will want to make use of VT-d/AMD-Vi.
Once you finish your thesis, I doubt you'll continue running the cloud
services (for which VT-d is maybe an afterthought), but you will want to
make the machine relevant for home use. If you prefer running a Linux
desktop, but you'd still like to play the latest games in a Windows VM
(where IOMMU shines), then there's no choice, but to go for something that
supports IOMMU. Note that using a machine at home usually implies that you
won't be needing ECC, so if you're fine with no ECC, then feel free to
disregard the caveat about Intel Core CPUs.


Yes, I thought that the AMD-Vi would be a good choice.
I would not want to have any obstacles with my VM running Windows for
home use and I also thought that IOMMU would make it easier for me to
learn Xen.

In reply to Adrian,

ECC is something that you may not notice when things are running well,
but if you are a company in a production environment, you do not want
to get corrupt memory causing you expensive problems, only to find out
weeks or months later when the memory finally goes and you see how
much this really cost you.
For my home use, it is only going to be peace of mind, etc., so like I
said above, if I can afford it, I will choose it. If I cannot afford
it, then non-ECC RAM will be ok for my purposes.
Thanks for the heads up on the power usage and noise. And the green
choices. Once I decide, I will consider all that as well.
I am not much of a gamer, but I may delve into some of those other
projects you mention.

Thanks everyone,

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