Author: Omid David Tabibi
Date: 02:48:41 03/29/03
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On March 29, 2003 at 03:49:21, Tom Kerrigan wrote: >I just finished a mess of tests, running my engine for 60 seconds/pos on a set >of 7 ECM positions that it solves in 30 to 60 seconds. > >I used the hash table sizes 1, 2, 4, 8, 16, 32, 64, 128, 256. > >The average depth being searched at 60 seconds was: >11.42, 11.71, 11.71, 11.71, 11.71, 11.71, 11.71, 11.71, 11.71 >So increasing hash table size beyond a couple of MB won't make you search >significantly deeper. > >The sum of times to solutions was: >361 (4/7 right), 342 (5/7), 329 (6/7), 321 (6/7), 310, 309, 309, 309, 313 >In other words, increasing the size up to 16MB makes a huge difference, but >there's no benefit to going bigger. > >The nodes/second times were: >1074k, 1074k, 1076k, 1077k, 1083k, 1084k, 1084k, 1079k, 1073k >So the biggest swing is 1%, so hash table size basically doesn't affect NPS. For >some reason speed seems to rise, peak at ~32MB, and fall. I can't explain that; >maybe it's just random. > >I was curious about how full the hash table was getting, so I wrote some code to >count empty entries after the search and also to count the number of overwrites, >i.e., when data for one position is overwritten by data for another position. >For reasons that should be pretty clear, I only calculated these numbers for the >depth-replace hash table and not the always-replace hash table. I only got >statistics for (IMO) the most significant hash table sizes. > >This is the hash table "fullness": >NA, NA, NA, 100%, 100%, NA, NA, NA, 44% > >This is the percentage of writes that were overwrites: >NA, NA, NA, 30%, 27%, NA, NA, NA, 4% > >It's interesting to note that the 16MB table is getting completely filled and 1 >in 4 writes are overwrites, i.e., a lot of information is being lost, but it >performs as well as the 256MB table which gets less than half full and hardly >any information is lost. > >In other words, it doesn't hurt to have a huge hash table, but anything more >than a surprisingly small table (~16MB = ~30% overwrites) likely isn't helping. >Contrary to Hyatt's assertion, it apparently doesn't matter if the hash table >fills up, or if it can hold the entire search tree. > Could you please mention which 7 ECM positions were you using? I will try to conduct your tests using Genesis, as it provides very detailed statistics regarding the hash table. Omid. >-Tom
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