Author: Andreas Guettinger
Date: 07:30:29 07/09/04
Go up one level in this thread
On July 09, 2004 at 09:43:32, Robert Hyatt wrote: >On July 09, 2004 at 09:30:56, Andreas Guettinger wrote: > >>On July 09, 2004 at 09:13:31, Robert Hyatt wrote: >> >>>On July 09, 2004 at 08:38:31, Tord Romstad wrote: >>> >>>>I am currently writing a chess engine. Parallel search is not among my main >>>>interests at the moment, but it is not entirely impossible that I will give it >>>>a try some time in the future. >>>> >>>>In order to keep everything as flexible as possible, I would like to design >>>>my algorithms and data structures in such a way that adding parallel search >>>>at a later stage is feasible. I understand that I should remove most of my >>>>global variables and replace them with huge structs containing the same data, >>>>and use one such struct for each thread. Is there anything else which is >>>>important to keep in mind? >>>> >>>>Tord >>> >>> >>>That's the main issue assuming you are going to use lightweight processes >>>(threads) which I believe is the best approach. The most thread-specific data >>>you have, which means less global data, will help performance (modified global >>>data is not cache-friendly on a SMP box) and simplify testing (since modified >>>global data requires atomic locks to avoid interleaved update problems). >> >>But crafty is using a global copy of the tree strcut (local[CPU]) per thread in >>parallel mode, isn't it? >> >>regards >>Andy > > >That's the idea. if you look, each "copy" of the tree struct is allocated >locally for NUMA boxes, so that it is on the actual processor where the thread >executes. > >But I think you are mixing terms. "local vs global" to a compiler person refers >to when the data "exists". IE global data exists when the program starts, while >local data is allocated on the stack and freed as needed. > >Here the idea is local data is only modified by a single thread, where global >data is modified by all. So it is not the same as when discussing the scope of >a variable in the context of a programming language. For parallel search it >doesn't matter whether the data is global, local, or malloc()'ed or whatever. >What matters is can my processor keep my data in its local cache without it >becoming invalidated due to another processor modifying it in its cache. If >that happens, the data is global/shared and is a problem. If that doesn't >happen, then it doesn't matter whether the data is local or global, it is not >shared and to the thread, it is local since it will exist in only one cache. Thanks, I have to think about that some more when I arrive at home. Still, the threads have to access the tree structs of other threads, i.e. when arriving at a split point. On the other hand each tree can be easily protected using a mutex. regards Andy
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