[fpc-pascal] Optimal number of threads for SMP

[Marco van de Voort]

In our previous episode, Xiangrong Fang said:
> If my application needs SPEED, i.e. take full advantage of CPU
> capabilities, and the application has no I/O operation at all (neither disk
> nor network), it seems no need to create threads more than the number of
> CPU cores ?hyper-threads)?
> 
> Am I right?

My rule of thumb is physical cores + a percentage. (like 10-20%).

The idea is to saturate the CPU, but only just so, since saturating by way
too many threads increasing switching time and thus decreases performance.

The percentage is a rule of thumb approach to account for some minor I/O (if
only OS background), scheduler/locks switch time,  and, possibly
hyperthreading.

But in my apps I always make it configurable, and use this sum only to seed
the default of the configuration setting.

This because you don't always want to fully utilize the computer (e.g.
to give some air to background services, or because you are working on it),
and because automatic detection regularly goes wrong with new CPUs etc.