Grokking concurrency, message passing and Co

[sybrandy]

>>>>> The rule of thumb is don't bother spawning more threads than you
>>>>> have cpus. You're just wasting resources mostly.
>>>>>
>>>> You REALLY don't want more threads trying to run than you have cores.
>>>> Threads in a wait state, are less of an issue, but they still use up
>>>> resources.

>
> Personally I'd never use more threads than cores as a program design,
> but I'm a performance whore. ;)
>

Could you explain why you believe this a bit more?  I'm curious because 
I've worked with databases and the rule of thumb there was you should 
configure them to use twice as many threads as number of cores/CPUs. 
The reasoning, I believe, is that if one thread on the CPU is stalled, 
another thread can execute in it's place.  And yes, I believe this was 
before hyperthreading.

Though I wasn't informed of the specifics as to why, I'm guessing it's 
mainly because with a database, there's a good chance you have to 
perform some sort of disk I/O.  Therefore instead of having the CPU wait 
while the I/O is occurring, another thread can start executing while the 
original is waiting.

Casey