Re: Virtualization Performance: Intel vs. AMD

[Gordan Bobic <gordan@bobich.net>]

Thomas Fjellstrom wrote:
> On Sun November 15 2009, Neil Aggarwal wrote:
>>> The Core i7 has hyperthreading, so you see 8 logical CPUs.
>> Are you saying the AMD processors do not have hyperthreading?
> 
> Course not. Hyperthreading is dubious at best.

That's a rather questionable answer to a rather broad issue. SMT is 
useful, especially on processors with deep pipelines (think Pentium 4 - 
and in general, deeper pipelines tend to be required for higher clock 
speeds), because it reduces the number of context switches. Context 
switches are certainly one of the most expensive operations if not the 
most expensive operation you can do on a processor, and typically 
requires flushing the pipelines. Double the number of hardware threads, 
and you halve the number of context switches.

This typically isn't useful if your CPU is processing one 
single-threaded application 99% of the time, but on a loaded server it 
can make a significant difference to throughput.

Gordan