Re: [Bug-gnubg] Optimal Setting for Eval Threads

[Philippe Michel]

On Wed, 12 Sep 2012, M. J. Mannon wrote:

> All the references I can find suggest setting "Eval Threads" to the 
> number of *cores* on your system. Most of these sources, I fear, refer 
> to CPUs that do not have hyperthreading.
>
> I have a 4-core CPU. With hyperthreading (each core runs 2 threads), it 
> has 8 threads.
>
> So, is the recommended setting for me 4 or 8?

Hyperthreading helps, but less than a real core. On the other hand 
multiple threads have a cost related to contention issues.

Here is what I get with a real quad-core :

threads normalized speed
1       1 * 100
2       2 *  91.6
3       3 *  89.3
4       4 *  87.1

and with a dual-core with HT :

1       1 * 100
2       2 *  91.3
3       3 *  69.5  or about 89 * ( 1 + 1 + 0.3 )
4       4 *  58.2           87 * ( 1 + 1 + 0.3 + 0.3 )

Your mileage may vary (this was a match analysis on an old Xeon processor, 
rollouts on a Core i-something may give different ratios), but I would 
guess in your case the 5th to 8th threads would give some (possibly modest 
for the last ones) improvement.

For an overnight job, I would use 8 threads ; if I was doing something 
else at the same time probably only 4.