If one set c=0, one must do something else to balancng the width and depth. If not, one will get very inconsistent results. By the way I'm not sure c is the best way to balancing the width and depth. This could be an excellent subject for research.
DL -----Original Message----- From: Brian Sheppard <sheppar...@aol.com> To: computer-go@computer-go.org Sent: Tue, 28 Apr 2009 8:31 am Subject: [computer-go] Justification for c==0 The Mogo team has published that their UCT "exploration coefficient" is zero, and further state that this is the optimal setting for Mogo. Other studies have confirmed that finding. Yet, the suspicion persists that this result is somehow related to Mogo's structure, perhaps because it runs massively parallel or because of some twist in its internals. ? This post provides theoretical and heuristic justification for c==0. First the theoretical: ? Theorem: In a finite game tree with no cycles,?with binary rewards, the UCT algorithm with c==0 converges (in the absence of computational limitations) to the game theoretic optimal policy. ? The proof is by induction on the depth of the tree. The base case is one ply before a terminal state. In the base case, UCT will eventually try a winning move if one exists. Thereafter, UCT will repeat that move indefinitely because there is no exploration. It follows that the UCT value of the base case will converge to the game theoretic value for both winning and losing states. For the induction step, assume that we have N > 1 plies remaining. Each trial produces a node at depth N-1 at most. (Note: for this to be true, we have to count ply depth according to the longest path to terminal node.) With sufficient numbers of trials, each of those nodes will converge to the game-theoretic value. This implies that if there is a winning play, it will eventually be discovered. ? Note that the "binary rewards" condition is required. Without it, the UCT policy cannot know that winning is the best possible outcome, so it would have to explore. ? The point of this theorem is that Mogo's is safe; its exploration policy does not prevent it from eventually playing perfectly. ? Now, there is no implication in this proof that the c==0 policy is computationally optimal, or even efficient. But we do have Mogo's experimental result, so it is worthwhile to speculate whether c==0 should be optimal. Some heuristic reasoning follows. ? If UCT has to choose between trying a move that wins 55% and a move that wins 54%, then why *shouldn't* it try the move that wins more frequently? What we are trying to do (at an internal node) is to prove that our opponent's last play was losing, and we would do this most efficiently by sampling the move that has the highest success. ? Another angle: at the root of the tree, we will choose the move that has the largest number of trials. We would like that to be a winning move. From the theorem above, we know that the value of the moves will converge to either 0 or 1. By spending more effort on the move with higher reward, we provide the maximum confirmation of the quality of the chosen move. If the reward of that move starts to drift downward, then it is good that we spent the effort. ? Another angle: we can spend time on either move A or move B, with A higher. If A is winning, then it is a waste of time to search B even?one time. So in that case c==0 is optimal. The harder case is where A is losing: we have spent more time on A and it has a higher win rate, so we would choose move A unless something changes. There are two strategies: invest in A to prove that it is not as good as it looks, or invest in B to prove that it is better than it seems. With only two move choices, these alternatives are probably about equal. But what if we had hundreds of alternatives? We would have a hard time guessing the winning play. So even when move A is losing we might be better off investing effort to disprove it, which would allow an alternative to rise. ? One more thought: Suppose that move A wins 56 out of 100 trials, and move B wins 5 out of 9. Which represents better evidence of superiority? Move A is more standard deviations over 50%. Does that suggest a new exploration policy? ? OK, so you don't have to worry if you set c==0. It might even be best. Just a note: in very preliminary experiments, c==0 is not best for Pebbles. If longer experiments confirm that, I presume it is because Pebbles runs on a very slow computer and searches only small trees. So your mileage may vary. But if c==0 tests well, then there is no reason not to use it. ? Best, Brian _______________________________________________ computer-go mailing list computer-go@computer-go.org http://www.computer-go.org/mailman/listinfo/computer-go/
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