Author: Don Dailey
Date: 20:19:24 01/17/98
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On January 17, 1998 at 18:44:07, Robert Hyatt wrote: >On January 17, 1998 at 13:07:14, Don Dailey wrote: > >>Bob, >> >>I am glad you cleared this up. Everyone does seem to think that once >>the hash table reaches saturation "everything backs up like a stopped >>up sink" and life is over. But as you stated this is not the case. >> >>A (very) rough guide is that once your hash table size reaches >>saturation >>you will get a 6% speedup if you double it. But the proper way to view >>this is as an extra BENEFIT not a bottleneck. If you double the search >>time your program will benefit TREMENDOUSLY, if you double the hash size >>it will benefit only SLIGHTLY. > >I just tried this, and didn't see this "tremendously" you mention. IE I >kept searching the same position deeper and deeper until after an >iteration >finished it reported the hash was 99% full. I cut it by half, and ran >to >the same depth, in roughly the same time... I tried this on three >positions >and one of the three slowed down by 3-4%. I don't think you understood my sentence. It almost sounds like you do not believe doubling the hardware speed is much of a benefit but that's what I was saying. To reword: It is a LOT more beneficial to double the time you spend searching than to double your hash table size. This paragraph was designed to back up your argument so don't try to refute it! About the 6% I get vs the 3% you get. This can probably be explained and is not a point of contention because I do get 6%. First of all the numbers will vary too much for only 3 positions to measure. Also it's possible you need more saturation (being barely saturated at 99% is not the same as having overwritten all the entries many times.) Try somewhat longer times. Also it could be cache effects that are sucking a little of the benefit. Finally it could be that your replacement scheme changes these numbers for you, (this ones smells funny though.) Node counts might be more accurate than times for this one. At any rate your numbers are in the rough ballpark so I don't think anything is wrong with your program. Larry noticed that most programs obey this rule of thumb. I forget where we heard this from originally but I believe it might have been Ken Thompson (but don't quote me) and it's accurate for us. We used to tune for tournaments too. I'm too lazy for this now but there was a time I would try to find the smallest hash table size I could get away with. We noticed if the table was much bigger than we needed the program was 2 or 3 percent slower! Now days we don't bother and use hash table aging anyway which makes this less appealing. - Don
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