RE: [Xen-users] AMD or Intel?

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

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> James Harper
> Sent: 24 May 2007 02:22
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] AMD or Intel?
> 
> We are looking at buying a server to do virtualisation on, 
> hopefully Xen
> assuming all my testing goes well.
> 
> Has anyone done any benchmarks on CPU's from both AMD and 
> Intel for Xen
> HVM scenarios? AMD's are a bit cheaper but in the line of 
> servers we are
> looking at, only the Intel processors come in a quad core 
> configuration,
> which would mean we could buy a single quad core processor now and
> upgrade later when required. With the AMD processors, there will be no
> room to upgrade if we start with 4 processors.

To quote from a previous post on the same subject from me: <quote>
As you can see from my mail-address, I'm certainly biased... 

However, wearing my patented "objective-tin-foil-hat" to make me a bit
more unbiased, I'll try to say this:
There is VERY LITTLE technical "external" differences between the AMD
and Intel virtualization technologies, so from that perspective one
isn't noticably better than the other. 
</quote>

I doubt that there is a huge difference between AMD and Intel on the
basis that there's very similar number of intercepts and the number of
clocks for a VMExit+Vmentry are roughly the same [1] - and the majority
of intercepts are:
- Page-fault - this is often the biggest intercept count by far, because
it's responsible for both MMIO and shadow-paging. 
- IOIO operations (disk, network, PIC, APIC, timers and other HW devices
- depends quite a bit on what the guest does).

Both of these type of intercepts are fairly complex to handle, as it
involves both interpreting the instructions that the guest was
executing, and then performing the adequate operations. For MMIO and
IOIO operations, it also often leads to qemu-dm calls. 

There are some minor differences between AMD and Intel in how and what
gets intercepted, but it's more on the "rare" side, rather than the
common ones. Intercepts that happen 1, 2 or 3% of the time isn't the
ones that make the big performance impacts!

So what I'm trying to say is that the difference in the implementation
of the virtualization in the two processors will be a minor part of the
overall performance of the virtual machine overall. 

There is, however, a major difference in the overall architecture, which
DOES affect the performance of the virtual machine: the AMD processors
have built-in memory controller, whilst the Intel products have an
external memory controller. This helps more on virtual machines than it
does on other processor applications, because all sorts of extra memory
requirements (hypervisor itself, multiple guests sharing the same
processor, shadow page-tables [that is, twice as much memory used for
page-tables], emulation of HW devices using a little bit of extra memory
- all of these add up a little bit at a time, compared to the bare-metal
solution). 

Admittedly, AMD doesn't have a quad-core today, but we're not THAT far
away[2] from releasing our quad-core products, so if you need to upgrade
later on, you should be able to replace your current dual-core(s) with
new quad-core processor(s) in the same socket(s). 

[1] From our internal measurement a while back, the AMD processor was a
tad faster - but that was a while back and it's not an official
benchmark that got published, just some measurements one of my collegues
did "on his spare time".

[2] I wish I could tell you exactly when we are releasing the new
generation processors, but for several reasons I can't tell you. The
major reason here is that I simply don't know, as I don't work for the
marketing department, but rather work for the engineering side where
processors and software components are developed. But if I did know, I
wouldn't be allowed to tell you anyways. 

--
Mats


> 
> All comments appreciated!
> 
> Thanks
> 
> James
> 
> 
> _______________________________________________
> Xen-users mailing list
> Xen-users@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-users
> 
> 
> 



_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users