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Subject: Re: The need to unmake move

Author: Robert Hyatt

Date: 10:08:34 09/03/03

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On September 03, 2003 at 09:02:28, Matthew Hull wrote:

>On September 03, 2003 at 00:07:37, Jeremiah Penery wrote:
>
>>On September 02, 2003 at 22:54:49, Robert Hyatt wrote:
>>
>>>Maybe.  But I use threads.  And on NUMA threads are _bad_.  One example,
>>>do you _really_ want to _share_ all the attack bitmap stuff?  That means it
>>>is in one processor's local memory, but will be slow for all others.  What
>>>about the instructions?  Same thing.
>>
>>After some thinking, it seems to me that the *average* memory access speed will
>>be the same no matter where the data is placed, for anything intended to be
>>shared between all processors (in a small NUMA configuration).  The reason for
>>this is because what is local to one processor will be non-local to all others.
>>It doesn't matter if everything is local to the same processor or spread around,
>>because the same percentage of total accesses will be non-local in any case
>>(unless there is a disparity between the number of accesses each CPU is trying
>>to accomplish).
>>
>>The only problem is that one processor's memory banks might get hammered, but
>>that _is_ the same with an (similarly small) SMP configuration - all accesses go
>>serially through one memory controller.
>>
>>As machine size increases, of course, NUMA can run into more problems.  But then
>>SMP has its own problems as well (cost and complexity of memory sub-system,
>>mostly).
>
>But for chess, each piece of the search process will be on it's own processor
>and all the data it needs to do the search will be copied once at the start of
>it's "assignment", then it's off to the races.  The only time he needs to use
>non-local memory is at the start (copy) and the end (report results).
>
>Yes?
>
>MH


Very close.  I also need to access some "shared" data at a split point, so that
I can be sure to search every move there once, but only once.  That is _very_
infrequent of course.  IE a typical position has 35 moves that need to be
searched.  A 4 ply search below that point dwarfs that overhead into nothing.



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