Re: [Xen-users] AMD or Intel?

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Petersson, Mats wrote:
>  
>
>   
>> -----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). 
>   
On a somewhat related topic, my understanding is that Barcelona/K10 will
provide hardware (IO-MMU?) that will allow expansion cards (PCI, AGP,
PCIe, etc.) to be virtualised. Currently, if an expansion card were to
attempt to access RAM in a guest it would be accessing the RAM at that
address in the host, i.e., memory accesses in guests are not mapped to
the appropriate memory in the host. Is it true that Barcelona/K10 will
provide functionality to solve this problem? If so, has any work been
done on supporting this hardware in Xen?

I would dearly love to reduce my impact on the environment via my energy
bill by plugging more video cards into my server in my cellar. :-)

Steve.


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