Young Brothers Wait Concept

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Young Brothers Wait Concept, (YBWC) a basic concept for a parallel alpha-beta search coined by Rainer Feldmann et al. in 1986, introduced 1989 to a wider audience in the ICCA Journal paper Distributed Game-Tree Search. Brothers are sibling nodes, the oldest brother is searched first, younger brothers, to be searched in parallel, have to wait until the oldest brother did not yield in a beta-cutoff and did possibly narrow the bounds in case of an alpha increase.

=Delay of Parallelism= The Young Brothers Wait Concept, which in some situations delays the use of parallelism until subtrees are available which are relevant for the final result with high probability, prevents processors from searching irrelevant subtrees and reduces the search overhead. The use of the YBWC is possible only in combination with good load balancing possibilities. The eldest son of any node must be completely evaluated before younger brothers of that node may be transmitted.

Distributed Game-Tree Search
In their introduction, Feldmann et al. mention approaches by several authors to decrease search overhead by various methods. The mandatory-work-first approach first evaluates that part of the game tree that must be searched and then evaluates the rest only if necessary. The Principal Variation Splitting (PVS) algorithm evaluates right sons of game-tree nodes with a minimal window and re-evaluates them only if necessary.

Alternatively, game-tree nodes are evaluated in parallel only if they had acquired an alpha-beta bound before. Yet another approach applies in a distributed chess program running on a hypercube : if the transposition table proposes some move for a game position, then this move is tried first. Parallel evaluation of the other moves is started only of the evaluation of the transposition move yields no cutoff.

Critique
Their 1989 ICCA paper was criticized by Jonathan Schaeffer, first on the speedup topic of about eleven on a 16-processor machine not mentioning that parallel speedup is tied to the (in)efficiency of the serial alpha-beta search, and further, on ignoring other work recently done in the field. The YBWC presented was similar to the methods used in the chess programs Cray Blitz, Lachex, and Para-Phoenix. In a response, Feldmann et al. agreed that the behavior of a parallelization should be measured relative to the "best" sequential program. However, the speedup depends on many implementation details such as those about the hardware, communication mechanism, load-balancing capabilities, etc., which cannot be described in a short paper. The authors further state that chess programs were not the main research vehicle, but the algorithm, which was the reason for not quoting all the chess specific papers.

=Quotes= Robert Hyatt on the distinction between ALL- and CUT-nodes : In CB, I used this extensively, because you never want to split at a CUT node, and want to prefer an ALL node. YBW tries to recognize an ALL node by requiring that at least one move be searched before splitting, since > 90% of CUT nodes are discovered on the first move searched. But waiting on that first move introduces some wait time, and if you knew it was an ALL node from the get-go, you could split before the first move is completed. That was the basic premise behind the iterated search and DTS algorithm in Cray Blitz...

=See also=
 * ABDADA
 * APHID
 * Dynamic Tree Splitting
 * Jamboree
 * Lazy SMP
 * Node Types
 * Parallelism and Selectivity in Game Tree Search | Video, Talk by Tord Romstad
 * Zugzwang (Program)

=Publications=

1986 ...

 * Rainer Feldmann, Peter Mysliwietz, Oliver Vornberger (1986). A Local Area Network Used as a Parallel Architecture. Technical Report 31, Paderborn University
 * Rainer Feldmann, Burkhard Monien, Peter Mysliwietz, Oliver Vornberger (1989). Distributed Game-Tree Search. ICCA Journal, Vol. 12, No. 2
 * Jonathan Schaeffer (1989). Comment on 'Distributed Game-Tree Search' . ICCA Journal, Vol. 12, No. 4

1990 ...

 * Rainer Feldmann, Burkhard Monien, Peter Mysliwietz, Oliver Vornberger (1990). Response to a Comment on 'Distributed Game-Tree Search' . ICCA Journal, Vol. 13, No. 1
 * Jonathan Schaeffer (1990). A Rejoinder to the Response to a Comment on 'Distributed Game-Tree Search'. ICCA Journal, Vol. 13, No. 1
 * Rainer Feldmann, Peter Mysliwietz, Burkhard Monien (1991). A Fully Distributed Chess Program. Advances in Computer Chess 6, pdf
 * Rainer Feldmann, Peter Mysliwietz, Burkhard Monien (1992). Experiments with a Fully Distributed Chess Program. Heuristic Programming in AI 3
 * Rainer Feldmann, Peter Mysliwietz, Burkhard Monien (1992). Distributed Game Tree Search on a Massively Parallel System. Data Structures and Efficient Algorithms, B. Monien, Th. Ottmann (eds.), Springer, Lecture Notes in Computer Science, 594, 1992, 270-288
 * Rainer Feldmann (1993). Game Tree Search on Massively Parallel Systems. Phd-Thesis, pdf
 * Rainer Feldmann, Peter Mysliwietz, Burkhard Monien (1994). Game-Tree Search on a Massively Parallel System. Advances in Computer Chess 7
 * Mark Brockington (1994). An Implementation of the Young Brothers Wait Concept. Internal report, University of Alberta

2010 ...

 * Kai Himstedt (2012). GridChess: Combining Optimistic Pondering with the Young Brothers Wait Concept. ICGA Journal, Vol. 35, No. 2 » GridChess, Pondering

=Forum Posts=

2009

 * YBWC details by Nathan Thom, CCC, December 31, 2009

2010 ...

 * DTS-ification of YBWC by Marco Costalba, CCC, June 01, 2010
 * YBWC: Active Reparenting by Marco Costalba, CCC, April 10, 2012

2015 ...

 * SMP questions by Patrice Duhamel, CCC, March 25, 2015
 * Actual speedups from YBWC and ABDADA on 8+ core machines? by Tom Kerrigan, CCC, July 10, 2015 » ABDADA
 * Lazy SMP Better than YBWC? by Steve Maughan, CCC, October 23, 2015 » Lazy SMP
 * NUMA in a YBWC implementation by Edsel Apostol, CCC, July 20, 2016 » NUMA
 * Some hyperthreading results by Kai Laskos, CCC, September 12, 2016 » Lazy SMP, Thread
 * YBWC implementation questions by Patrice Duhamel, CCC, August 26, 2017
 * More questions about threads by Patrice Duhamel, CCC, April 21, 2018 » Thread

=External Links=
 * AG-Monien - Research - Game Trees » Burkhard Monien
 * Younger Brother (disambiguation) from Wikipedia
 * Big Brother (disambiguation) from Wikipedia
 * Jan Garbarek Group - Storebror og Lillebror / Conversation, Festspillene in Bergen, Norway, May 25, 2002, YouTube Video
 * Jan Garbarek, Rainer Brüninghaus, Eberhard Weber, Marilyn Mazur

=References=

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