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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 5 Apr 2006, at 15:07, Magenheimer, Dan (HP Labs Fort Collins) wrote:

> I believe ppc has "paravirtualized spinlocks" in their Linux
> kernel, though even this won't necessarily help with a poorly
> written SMP application.
>
> No data, admittedly, but perhaps our good buddies at
> Watson could comment?
IBM did some investigation into this a while back. The conclusion then 
was that, even in a microbenchmark somewhat contrived to try and 
indicate 'worst case' spinlock behaviour, the benefit of smarter 
spinlocks was negligible. Maybe the benchmark was bad, maybe there are 
other pathological worst cases out there that the benchmark did not 
represent, or maybe the smart spinlocks should have been smarter. 
Whatever: the numbers we have so far do not indicate that there's a 
problem to be solved, and these potential multi-processor optimisations 
have to be empirically driven because intuition is so frequently off 
the mark.
 -- Keir


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