[Bug-gnubg] Re: MT/SMP bug disappeared??!

[Jonathan Kinsey]

Achim Mueller wrote:
> Hi folks,
> 
> weird things are going on. I made a few tests with a certain position
> regarding the MT bug (standard error no/double).
> 
> I played a moneygame (attached, it's not that bad ;-) ) and made the
> following rollouts at the redouble/take position:
> 
> Current snapshot, all rollouts 2-ply, cube 2ply, 108 games, untruncated, 
> dice Mersenne Twister, seed 1, I restarted gnubg after each rollout!
> 
> 1) Twice at the take with one processor
> 2) Twice at the take with two processors
> 3) Twice at the double with one processor
> 4) Twice at the double with two processors
> 
> Here are the numbers:
> 
>         nd      std.err   take    std.error
> 
> 1a      +0.569  0.013     +0.654    0.033
> 1b      exactly the same results (what I expected) 
> 
> 2a      +0.569  0.046     +0.654    0.033
> 2b      +0.569  0.045     +0.654    0.033 
> 
> 3a      exactly the same as 1a and 1b
> 3b      exactly the same as 1a and 1b
> 
> 4a      +0.569  0.048     +0.654    0.033
> 4b      +0.569  0.046     +0.654    0.033
> 
> Question 1: The std.error is not shown as big as it was when I first
> noticed it, but it's still a little bit higher than with using only one
> processor. Why?
> 
> Question 2: It differs! Why? I expected the same numbers because of the
> same seed.

Very small differences are very possible as the multiple threads share the
random number generator, in practice this is unimportant.

Are you running this on linux or windows?

If I get some time, I'll try to debug the original problem - there may well be a
problem with the std-error calculation code.

Jon

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