Shared Hash Table

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Dining philosophers problem [1]

Shared Hash Table,
a Hash table or Transposition table which is accessed by various processes or threads simultaneously, running on multiple processors or processor cores. Shared hash tables are most often implemented as dynamically allocated memory treated as global array. Due to memory protection between processes, they require an Application programming interface provided by the operating system to allocate shared memory. Threads may share global memory from the process they are belonging to.

Parallel Search

Almost all parallel search algorithms on SMP- or NUMA systems profit from probing hash entries written by other instances of the search, in its most simple form by instances of a sequential search algorithm which simultaneously search the same root position. The gains come from the effect of nondeterminism. Each processor will finish the various subtrees in varying amounts of time, and as the search continues, these effects grow making the search trees diverge. The speedup is then based on how many nodes the main processor is able to skip from transposition table entries. It had the reputation of little speedup on a mere 2 processors, and to scale quite badly after this. However, the NPS scaling is nearly perfect.


see Main page: ABDADA

ABDADA, Alpha-Bêta Distribué avec Droit d'Anesse (Distributed Alpha-Beta Search with Eldest Son Right) is a loosely synchronized, distributed search algorithm by Jean-Christophe Weill [2] . It is based on the Shared Hash Table, and adds the number of processors searching this node inside the hash-table entry for better utilization - considering the Young Brothers Wait Concept.

Lazy SMP

see Main page: Lazy SMP

Recent improvements by Daniel Homan [3], Martin Sedlak [4] and others on Lazy SMP indicate that the algorithm scales quite well up to 8 cores and beyond [5].

Concurrent Access

Due to its size, i.e. 16 or more bytes, writing and reading hash entries are none atomic and require multiple write- and read-cycles. It may and will happen that concurrent writes and reads at the same table address and almost same time results in corrupt data retrieved that causes significant problems to the search. Interrupts may occur between accesses, and there are further nondeterministic issues involved [6] which may cause one thread to read two or more atomic data items, which were written by different threads, searching different positions with the same hash-index due to type-2 errors.


One common solution to avoid such errors is synchronization using atomic locks, and to implement a critical section or mutual exclusion.


As an example, CilkChess used Cilk's support for atomicity [7]. It uses one lock per hash entry:

typedef struct
   Cilk_lockvar lock;
   U64 key;
   U64 data;
} ttentry;

ttentry hashtable[TABLESIZE];

void ttinit ( ) {
   for (int i = 0; i < TABLESIZE; ++i)
      Cilk_lock_init( hashtable[i].lock);

void update_entry ( ttentry *e, U64 key, U64 data ) {
   Cilk_lock (e->lock); /* begin critical section */
   e->key = key;
   e->data = data;
   Cilk_unlock (e->lock); /* end critical section */


An important property of a lock is its granularity, which is a measure of the amount of data the lock is protecting. In general, choosing a coarse granularity (a small number of locks, each protecting a large segment of data) results in less lock overhead when a single process is accessing the protected data, but worse performance when multiple processes are running concurrently. This is because of increased lock contention. The more coarse the lock, the higher the likelihood that the lock will stop an unrelated process from proceeding, i.e. in the extreme case, one lock for the whole table. Conversely, using a fine granularity (a larger number of locks, each protecting a fairly small amount of data), like in the CilkChess sample above, increases the overhead of the locks themselves but reduces lock contention. For a huge transposition table with millions of fairly small entries locks incur a significant performance penalty on many architectures.



Robert Hyatt and Tim Mann proposed a lock-less transposition table implementation [8] for 128 bit entries with two atomic quad words, one qword for storing the key or signature, the 64-bit Zobrist- or BCH-key of the position, and one qword for the other information stored, move, score, draft and that like (data). Rather than to store two disjoint items, the key is stored xored with data, while data is stored additionally as usual. According to Robert Hyatt, the original idea came from Harry Nelson somewhere in 1990-1992 [9].

index = key % TABLESIZE;
hashtable[index].key  = key ^ data;
hashtable[index].data = data;

Since the retrieving position requires the same key for a probing hit, the stored key xored by the retrieved key must match the stored data.

index = key % TABLESIZE;
if (( hashtable[index].key ^ hashtable[index].data) == key )
   /* entry matches key */

If key and data were written simultaneously by different search instances with different keys, the error will usually yield in a mismatch of the comparison, except the rare but inherent Key collisions or type-1 errors [10]. As pointed out by Harm Geert Muller [11], the XOR technique might be applied for any size.


For a lock-less shared Hash table with (much) larger entry sizes such as the Pawn Hash Table, one may store an additional checksum of the data, to likely detect errors after retrieving, and to safe the consistence of an entry.


x86 and x86-64 SSE2 128-bit read/write instructions might in practice atomic, but they are not guaranteed even if properly aligned [12] [13]. If the processor implements a 16-byte store instruction internally as 2 8-byte stores in the store pipeline, it's perfectly possible for another processor to "steal" the cache line in between the two stores [14]. However, Intel states any locked instruction (either the XCHG instruction or another read-modify-write instruction with a LOCK prefix) appears to execute as an indivisible and uninterruptible sequence of load(s) followed by store(s) regardless of alignment [15] [16].


Multiple threads inside one process can share its global variables or heap. Processes require special API calls to create shared memory and to pass a handle to other processes around for interprocess communication. POSIX provides a standardized API for using shared memory [17] . Linux kernel builds since 2.6 offer /dev/shm as shared memory in the form of a RAM disk.

See also



1980 ...

1990 ...

Abstract: The method of parallelization is based on a suppression of control between the search processes, in favor of a speculative parallelism and full sharing of information achieved through a physically distributed but virtually shared memory. The contribution of our approach for real-time distributed systems and fault-tolerant is evaluated through experimental results.

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Forum Posts

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External Links

Shared Memory

Shared Memory:

False sharing from Wikipedia



MSI protocol from Wikipedia
MESI protocol from Wikipedia
MOESI protocol from Wikipedia
assembly - The prefetch instruction - Stack Overflow
Data Prefetch Support - GNU Project - Free Software Foundation (FSF)
Software prefetching considered harmful by Linus Torvalds,, May 19, 2011

Concurrency and Synchronization


Cooperating sequential processes (EWD 123)
A challenge to memory designers? (EWD 497)

Distributed memory

Koorde based on Chord and De Bruijn Sequence



  1. Dining philosophers problem from Wikipedia
  2. Jean-Christophe Weill (1996). The ABDADA Distributed Minimax Search Agorithm. Proceedings of the 1996 ACM Computer Science Conference, pp. 131-138. ACM, New York, N.Y, reprinted ICCA Journal, Vol. 19, No. 1, zipped postscript
  3. Lazy SMP, part 2 by Daniel Homan, CCC, January 12, 2013
  4. Lazy SMP in Cheng by Martin Sedlak, CCC, February 02, 2015
  5. Re: A new chess engine : m8 (comming not so soon) by Peter Österlund, CCC, February 01, 2015
  6. Memory disambiguation from Wikipedia
  7. Don Dailey, Charles E. Leiserson (2001). Using Cilk to Write Multiprocessor Chess Programs. Advances in Computer Games 9, pdf, 5 Other parallel programming issues, Cilk support for atomicity, pp. 17-18
  8. Robert Hyatt, Tim Mann (2002). A lock-less transposition table implementation for parallel search chess engines. ICGA Journal, Vol. 25, No. 1
  9. Re: Lockless hash: Thank you Bob Hyatt! by Robert Hyatt, CCC, August 26, 2013
  10. Robert Hyatt, Anthony Cozzie (2005). The Effect of Hash Signature Collisions in a Chess Program. ICGA Journal, Vol. 28, No. 3
  11. Re: lockless hashing by H.G.Muller, CCC, February 07, 2011
  12. Re: Effectively atomic read of 16 bytes on x86_64 without cmpxchg16b? by Anthony Williams,, February 08, 2012
  13. Re: Speaking of the hash table by Ronald de Man, CCC, December 10, 2012
  14. SSE instructions: single memory access by Atom, Stack overflow, October 04, 2011
  15. Intel® 64 and IA-32 Architectures Developer's Manual: Vol. 3A, section
  16. Fetch-and-add from Wikipedia
  17. shm_open, The Single UNIX Specification version 2, Copyright © 1997 The Open Group
  18. Maged Michael - Selected Publications
  19. The Aurora Distributed Shared Data System
  20. Re: scorpio can run on 8192 cores by Daniel Shawul, CCC, August 29, 2015
  21. Transposition-driven scheduling - Wikipedia
  22. Transposition driven scheduling by Daniel Shawul, CCC, April 04, 2013
  23. Information on the C++11 Memory Model by Scott Meyers, April 24, 2012
  24. John Romein, Henri Bal, Jonathan Schaeffer, Aske Plaat (2002). A Performance Analysis of Transposition-Table-Driven Scheduling in Distributed Search. IEEE Transactions on Parallel and Distributed Systems, Vol. 13, No. 5, pp. 447–459. pdf

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