TalkChess.com

[Daniel Shawul]

I found an interesting bug in the distributed tt a couple of days ago and the above post I made examplifies it. Here is the bug just in case someone finds it interesting too. I used a modulo on the hash key to find which processor has the section of the TT, but then I used the same hash key to do the probing as well. This effectively throws away most of the hash tables when many processors are used! about 90% of TT with 8 threads, 75% of TT with 4 threads are useless...The problem is that using a modulo of the hash_key to select which processor is going to store that position effectively makes processor 1 to store hash_keys ending in 1, processor 2 in 2, processor 3 in 3 etc... So only 1/N of the TT is used with N processors. To avoid that a division by N should be applied before the key is used for probing. It may not be necessarily a division that is applied. In code: