Re: [PATCH 1/10] Cr4 is valid on some 486s

[Zachary Amsden <zach@vmware.com>]

Arjan van de Ven wrote:

>On Mon, 2005-11-14 at 11:46 -0800, Zachary Amsden wrote:
>
>  
>
>>It seems that SMP vs. UP lock / spinlock overhead is relevant even for 
>>future, multi-core CPUs in a virtualization context, as the notion of 
>>hotplug here is based on scheduling constraints of the virtualization 
>>engine, and the kernel can quite readily end up with only one VCPU.
>>    
>>
>
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>this assumes that you don't just always want to assume and use SMP
>primitives in a virtualized context. I sort of question that assumption;
>sure these things have overhead, especially "lock", but if the solution
>is more complexity and weird things to hide that half-percent or less of
>performance difference... then do remember that such complexity is not
>free either. Runtime tricks cost. 
>  
>

Runtime tricks that increase complexity cost, yes.  It's all a question 
of measured gain vs. complexity.  But a couple of percent gained on an 
overall basis can be magnified enormously if you are looking at a 
workload that stresses a particular path.  I would expect some of those 
gains to be non-trivial, especially if considering the optimizations you 
could do on page table updates knowing you needn't worry about SMP 
issues anymore.  Even UP has (still?) some places where additional locks 
are present here, and could benefit from having SMP alternatives.

Zach