Author: Robert Hyatt
Date: 10:08:34 09/03/03
Go up one level in this thread
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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