Re: [Bug-gnubg] Threaded rollouts and RNGs

[Philippe Michel]

On Wed, 1 Feb 2017, Øystein Schønning-Johansen wrote:

> The rollout code in GNU Backgammon is of course threaded. How is this
> threaded actually? Is it threaded on each sample (aka each game). One
> thread pr. game? Or one thread pr. evaluation? Or?

I'm not sure I really understand how it is done, but the threads are fed 
with tasks and for a rollout I think a task is one playout of a candidate 
move. If some candidates are stopped on a standard deviation criteria, the 
others are then processed faster.

> I just wrote a code that does a threaded MC simulation of a different
> problem, and I noticed that I got significantly different results depending
> on either I used threading or not. Funny thing, it didn't initially occur
> to me, but of course I need one separate RNG instance for each thread.
> Fixing that didn't only give me the right results of the simulation, but it
> also improved the performance.
>
> How about GNU Backgammon? Do you use one separate RNG instance for each
> thread?

This is the principle, but it is complicated by the need to have each dice 
combination appear once for every 36 rolls.

Moreover, the UI is quite fragile since one can set the rng to something 
not thread-safe like "file" or "random.org". The defaut mersenne rng is 
ok, though. With it, for a given position and seed, the result is the same 
for any numbers of threads.