Dynamic Tree Splitting

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Dynamic Tree Splitting, (DTS) a peer-to-peer model of parallelism which divides the search tree among several processors on a shared memory parallel machine, devised by Robert Hyatt, implemented in Cray Blitz and tested on a 16 processor Cray C916/1024 with 1024 mebiwords of memory (8 gibibytes).DTS overcomes one problem of principle variation splitting (PVS) and enhanced principle variation splitting (EPVS) with bushy trees, that processors assigned to a node become and stay idle while other processors are still busy on the same node. In DTS, busy processors publish their state of the tree in shared memory for idle processors to examine, to let them decide which (if any) of the busy processors to join, and to inform a busy processor of the chosen expceted all-node or pv-node, dubbed split point. The busy processor with that subtree in progress in turn, then copies the complete tree state with current board position, bounds, move list, repetition list and so forth to a shared memory area, becoming a so called "split block". Owner and helper processors can now extract moves from this shared data to search in parallel.

=Performance Test= Rather than to use a group of unrelated chess problems, the CB team took a segment of a real chess game from the ACM 1993 M-Chess Pro vs. Cray Blitz, a sharp Vienna game aka delayed kings gambit accepted after CB was out of book after 1. e4 e5 2. Nc3 Nc6 3. f4 exf4 4. Nf3 g5 5. d4 g4 6. Bc4 gxf3 7. o-o d5 8. exd5 Bg4 9. Qd2, 24 positions in total. The results include two sets of numbers for each test, where a test used either one, two, four, eight or sixteen processors. The numbers are the clock time required for the search (wall clock time) and the number of nodes searched which shows how much extra work is added by using additional processors.

DTS Speedups
DTS Cray Blitz speedups on Cray C916/1024 over 24 test positions

Despite Robert Hyatt mentioned DTS really will only scale as far as a shared memory architecture scales, and any delays (such as those in some architectures that use a hierarchical memory organization with varying delays depending on how far the memory is from the requesting processor) will certainly have an adverse effect on DTS, the published speedups were heavily criticized by Vincent Diepeveen in 2002.

Comparison
Average Cray Blitz speedups on Cray C916/1024 over 24 test positions

=Recursive DTS= While searching various subtrees with different ply levels from the root was quite straighforward to implement in Cray Blitz' iterative search written in Fortran, a recursive Negamax would have made it much more difficult, since the call stack is inaccessible to the search code and moving the tree state around would have been more difficult. A recursive DTS-like algorithm was proposed by Peter Österlund in 2013, as implemented in Texel.

=See also=
 * ABDADA
 * Jamboree
 * Lazy SMP
 * NUMA
 * SMP
 * Young Brothers Wait Concept

=Publications=
 * Robert Hyatt (1994). The DTS high-performance parallel tree search algorithm.
 * Robert Hyatt (1997). The Dynamic Tree-Splitting Parallel Search Algorithm. ICCA Journal, Vol. 20, No. 1
 * Valavan Manohararajah (2001). Parallel Alpha-Beta Search on Shared Memory Multiprocessors. Masters Thesis, pdf
 * David J. Wu (2015). Designing a Winning Arimaa Program. ICGA Journal, Vol. 38, No. 1, pdf » Arimaa

=Forum Posts=
 * DTS article robert hyatt - revealing his bad math by Vincent Diepeveen, CCC, September 03, 2002
 * DTS NUMA by Vincent Diepeveen, CCC, September 03, 2002 » NUMA


 * Iterative DTS by Fritz Reul, CCC, July 02, 2007
 * DTS Structure by Edmund Moshammer, CCC, May 28, 2010 » Dynamic Tree Splitting, Iterative Search
 * DTS-ification of YBWC by Marco Costalba, CCC, June 01, 2010
 * Parallelization questions, ABDADA or DTS? by Benjamin Rosseaux, CCC, March 23, 2012 » ABDADA
 * Recursive DTS-like search algorithm by Peter Österlund, CCC, July 24, 2013 » Texel
 * DTS-like SMP by ThinkingALot, OpenChess Forum, July 25, 2013 » Gull
 * Dynamic Tree Splitting by Syed Fahad, CCC, March 13, 2015
 * Empirical results with Lazy SMP, YBWC, DTS by Kai Laskos, CCC, April 16, 2015 » Lazy SMP, YBWC

=External Links=
 * The DTS high-performance parallel tree search algorithm by Robert Hyatt
 * Porcupine Tree - Octane Twisted (2012), recorded at Riviera Theatre, Chicago, April 30, 2010, YouTube Video

=References=

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