Does dom0 see all physical processors? (RE: [Xen-ia64-devel] SAL INFO virtualization)

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Magenheimer, Dan (HP Labs Fort Collins)
>[mailto:dan.magenheimer@xxxxxx]
>Sent: 2006年4月3日 23:55
>>
>> Just an unrelated comment, Dom0 should always run on every
>physical
>> processor, which is the base requirement for performance
>> reason. It's not
>> "therefore" result of this specific issue and instead you can
>> take it as the
>> basic assumption for your thought. :-)
>>
>> Thanks,
>> Kevin
>
>Hmmm... are you saying that on a 16-processor Xen system,
>domain0 will run as a 16-way SMP guest?

OK, seems that I didn't describe the case clearly. :-) For current xen
model, dom0 default runs on all physical cpus which is however 
configurable.:

    if ( opt_dom0_max_vcpus == 0 )
        opt_dom0_max_vcpus = num_online_cpus();
    if ( opt_dom0_max_vcpus > MAX_VIRT_CPUS )
        opt_dom0_max_vcpus = MAX_VIRT_CPUS;
    printk("Dom0 has maximum %u VCPUs\n", opt_dom0_max_vcpus);

    for ( i = 1; i < opt_dom0_max_vcpus; i++ )
        (void)alloc_vcpu(d, i, i); /* vcpu N runs on physical cpu N */

In this case, max vcpus of dom0 is configurable but each vcpu is running 
at different physical processor once allocated. For a typical SMP box, 
such model can provide better performance or else dom0 will become the 
bottle neck if only owning one physical processor.

Then consider your question about a large box with many processors. 
How about the real environment? Is it the case to provide a 16-way SMP 
box, or a 16-way NUMA box? I prefer to the latter. If it's a NUMA box, 
dom0 sees physical ACPI table and can be configured as NUMA aware.

Yes, such binding model may change later but it's always bad to have 
dom0 only running on single physical processor.

CC to a wider list for comments. :-)

Thanks,
Kevin

>
>This isn't the case today with Xen (x86 or ia64) and I
>don't expect that it will work this way in the future
>either.
>
>Dan

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