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.
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
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