Re: [Xen-devel] Windows SMP

[Steve Ofsthun <sofsthun@xxxxxxxxxxxxxxx>]

Venefax wrote:
> Dear Gentlemen

> I have a machine with Intel 7350, 4 quad core. I can test since I have two
> virtual machines with 8 vcpus, showing a gross overhead that makes them
> unsuited for business. I use SLES 10 SP2, and I don't know how to apply the
> patch, but if somebody can log in and apply it, we can see the results
> immediately. The issue is affecting me directly. My two VM's have a VOIP
> application, very intensive in network and CPU usage.

If your problem isn't the previously discussed TPR issues ...

What Windows version are you running in your guests?

Windows 2000 SMP has serious problems due to the CPU wasting idle loop.  We 
avoid this with a special idler daemon running in the guest.


Are you over-committing VCPUS at all (more than one active VCPU per CPU)?

We have noticed significant performance degradation with SMP windows once you 
over-commit VCPUS on the host.  This seems to be due to excessive guest 
spinlock overhead caused by the spinning VCPUS wasting their entire quantum 
looping for the guest lock at the same time they are preventing the guest lock 
holding VCPU from running.  We have been experimenting with new CPU features 
for exiting on PAUSE instructions.  This can possibly be used to detect the CPU 
wasting spinners.


Have you tried using the Novell shim (Vista/2008 guests)?

I'm not sure the shim provides locking enhancements, but we have seen benefits 
for certain workloads.


Are you running PV on HVM drivers?

One last area of concern may be distributing callback interrupts to all VCPUS.  
We still run Xen (SLES10 SP2 based) with a patch to force all guest callback 
interrupts to VCPU 0.  This has consistently improved our SMP guest I/O 
performance while running PV-on-HVM drivers.


Steve

> Federico
> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx] 
> Sent: Wednesday, December 31, 2008 5:10 AM
> To: Andrew Lyon
> Cc: Venefax; James Harper; Dirk Utterback; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Windows SMP
> 
> On 31/12/2008 10:08, "Andrew Lyon" <andrew.lyon@xxxxxxxxx> wrote:
> 
>> Thanks Keir, I added a couple of printk's as you suggested and I can
>> now see if the feature is supported:
>>
>> Intel(R) Xeon(R) CPU           E5420  @ 2.50GHz = (XEN) APIC Access
>> virtualized
>> Intel(R) Core(TM)2 CPU          6600  @ 2.40GHz = (XEN) APIC Access
> emulated
>> I didnt expect my Xeon system to support it, I've read some pdf's
>> about Intel virtualizaton features and they seemed to suggest it was a
>> new feature on 7xxx Xeons.
>>
>> The results fit the performance I've seen, a windows xp 32 bit hvm
>> with 2 cpu's runs a lot faster on the Xeon 2.5 than on the Core 2.4.
> 
> I think it is probably worth printing out. I'll add a patch to xen-unstable.
> 
>  Thanks,
>  Keir
> 
> 
> 
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