Hi,
recently, Leela-Chess-Zero has become very strong, playing
on the same level with Stockfish-10. Many of the test players
are puzzled, however, by the "phenomenon" that Lc0 tends to
need many many moves to transform an overwhelming advantage
into a mate.
Just today a new German tester reported a case and described
it by the sentence "da wird der Hund in der Pfanne verrückt"
("now the dog is going crazy in the pan", to translate it word
by word). He had seen an endgame: Stockfish with naked king,
and LeelaZero with king, queen and two rooks. Leela first
sacrificed the queen, then one of the rooks, and only then
started to go for a "normal" mate with the last remaining rook
(+ king). The guy (Florian Wieting) asked for an explanation.
http://forum.computerschach.de/cgi-bin/mwf/topic_show.pl?tid=10262
I think there is a very straightforward one: What Leela-Chess-Zero
with its MCTS-based searc) performs is comparable to the
path all MCTS Go bots took for many years when playing winning
positions against human opponents: the advantage was reduced
step by step, and in the end the bot gained a win by 0.5 points.
Later, in the tournament table, that was not a problem, because
a win is a win :-)
Similarly in chess: overwhelming advantage is reduced by lazy play
to some small margin advantage (against a straightforward alpha-beta
opponent), and then the MCTS chess bot (= Leela Zero in this case)
starts playing concentratedly.
Another guy asked how DeepMind had worked around this problem
with their AlphaZero. I am rather convinced: They also had this
problem. Likely, they kept the most serious examples undisclosed,
and furthermore set the margins for resignation rather narrow (for
instance something like evaluation +-6 by Stockfish for three move
pairs) to avoid nearly endless endgames.
Ingo.
PS: thinking of a future with automatic cars in public traffic. The
0.5-point wins or the related behaviour in MCTS-based chess would mean
that an automatic car would brake only in the very last moment
knowing that it will be sufficient to stop 20 centimeters next to the
back-bumpers of the car ahead. Of course, a human passenger would
not like to experience such situations too often.
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