Author: Dann Corbit
Date: 13:33:10 08/08/02
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On August 08, 2002 at 16:17:37, Gian-Carlo Pascutto wrote: >On August 08, 2002 at 16:16:17, Aaron Gordon wrote: > > >>Indeed. This is why I am stressing people have high memory bandwidth so they >>can have large hash tables w/o taking much of a performance hit. > >Why would high memory bandwith have any relation to the performance >of large hashtables? > >Regardless of the size of the hashtable, the amount of data is constant >and very small. Most hashtable hits will not be in cache. That is the tragedy of hashing. In fact, if similar (but different) positions have similar hash values, then your hash is probably severely broken. Therefore, every hash lookup will be a probe of main memory (and every hash store a write to main memory). The on-CPU cache will never save you. Therefore, if you do a million-million probes in a long search and a similar number of stores, the memory speed is going to be very, very important. It's not important in blitz. It is important in real chess. Most of the top chess engines I analyze with a profiler have the hash function as the major bottleneck. And when you work with an engine for which that is not the case, it often ends up that way when you remove the other bottlenecks.
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