Re: KVM Processor cache size

Anthony Liguori
2010-08-02T23:36:18Z

On 08/02/2010 05:42 PM, Andre Przywara wrote:
> Anthony Liguori wrote:
>> On 08/02/2010 08:49 AM, Ulrich Drepper wrote:
>>> glibc uses the cache size information returned by cpuid to perform
>>> optimizations. For instance, copy operations which would pollute too
>>> much of the cache because they are large will use non-temporal
>>> instructions. There are real performance benefits.
>>
>> I imagine that there would be real performance problems from doing
>> live migration with -cpu host too if we don't guarantee these values
>> remain stable across migration...
> Again, -cpu host is not meant to be migrated.

Then it needs to prevent migration from happening. Otherwise, it's a
bug waiting to happen.

> There are other virtualization use cases than cloud-like server
> virtualization. Sometimes users don't care about migration (or even
> the live version), but want full CPU exposure for performance reasons
> (think of virtualizing Windows on a Linux desktop).
> I agree that -cpu host and migration should be addressed, but only to
> a certain degree. And missing migration experience should not be a
> road blocker for -cpu host.

When we can reasonably prevent it, we should prevent users from shooting
themselves in the foot. Honestly, I think -cpu host is exactly what you
would want to use in a cloud. A lot of private clouds and even public
clouds are largely based on homogenous hardware.

I actually think the case where you want to migrate between heterogenous
hardware is grossly overstated.

Regards,

Anthony Liguori

>
> Regards,
> Andre.
>