On 20/07/2012 12:45 AM, Darren New wrote:
> On 7/5/2012 11:22, Orchid Win7 v1 wrote:
>> (In spite of the fact that I'm actually using ALL the cores available.)
>
> You're not, tho. Hyperthreading means you have like 1.5 cores for every
> "core". I assume you know what hyperthreading does. You could probably
> see a speed-up by running 5 copies of pov-ray instead of 4.

As I understand it, hyperthreading allows you to schedule 2 threads on 
each core. The first thread gets all available resources. If any are 
unused, they are assigned to the second thread.

This means that if, say, one thread is using floating-point arithmetic 
and the other is using integer arithmetic, both threads run at nearly 
full speed. On the other hand, if both threads are using integer 
arithmetic, only one thread runs, and the other makes no progress. (At 
least, until the OS reschedules.)

Thus, I can believe that running two different applications might see a 
benefit. If they're performing different tasks, they probably use 
different execution unit types. But running two copies of the same 
application is highly unlikely to see any benefit - unless the 
application uses a really variable mix of instruction types...

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