[Computer-go] action-value Q for unexpanded nodes

Álvaro Begué alvaro.begue at gmail.com
Wed Dec 6 07:06:08 PST 2017

My hand-wavy argument succumbs to experimental data. And to a better
argument. :)

I stand corrected.


On Wed, Dec 6, 2017 at 8:52 AM, Gian-Carlo Pascutto <gcp at sjeng.org> wrote:

> On 06-12-17 11:47, Aja Huang wrote:
> > All I can say is that first-play-urgency is not a significant
> > technical detail, and what's why we didn't specify it in the paper.
> I will have to disagree here. Of course, it's always possible I'm
> misunderstanding something, or I have a program bug that I'm mixing up
> with this.
> Or maybe you mean that you expect the program to improve regardless of
> this setting. In any case, I've now seen people state here twice that
> this is detail that doesn't matter. But practical results suggest
> otherwise.
> For a strong supervised network, FPU=0 (i.e. not exploring all successor
> nodes for a longer time, relying strongly on policy priors) is much
> stronger. I've seen this in Leela Zero after we tested it, and I've
> known it to be true from regular Leela for a long time. IIRC, the strong
> open source Go bots also use some form of progressive widening, which
> produces the same effect.
> For a weak RL network without much useful policy priors, FPU>1 is much
> stronger than FPU=0.
> Now these are relative scores of course, so one could argue they don't
> affect the learning process. But they actually do that as well!
> The new AZ paper uses MCTS playouts = 800, and plays proportionally
> according to MCTS output. (Previous AGZ had playouts = 1600,
> proportional for first 30 moves).
> Consider what this means for the search probability outputs, exactly the
> thing the policy network has to learn. With FPU=1, the move
> probabilities are much more uniform, and the moves played are
> consequentially much more likely to be bad or even blunders, because
> there are less playouts that can be spent on the best move, even if it
> was found.
> > The initial value of Q is not very important because Q+U is
> > dominated by the U piece when the number of visits is small.
> a = Q(s, a) + coeff * P(s,a) * (sqrt(parent->visits) / 1.0f +
> child->visits());
> Assume parent->visits = 100, sqrt = 10
> Assume child->visits = 0
> Assume P(s, a) = 0.0027 (near uniform prior for "weak" network)
> The right most side of this (U term) is ~1. This clearly does not
> dominate the Q term. If Q > 1 (classic FPU) then every child node will
> get expanded. If Q = 0 (Q(s, a) = 0) then the first picked child
> (largest policy prior) will get something like 10 expansions before
> another child gets picked. That's a massive difference in search tree
> shape, *especially* with only 800 total playouts.
> --
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