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Subject: Re: Let's analyze move 36

Author: Robert Hyatt

Date: 18:05:03 10/08/97

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On October 08, 1997 at 14:08:42, Amir Ban wrote:

>On October 08, 1997 at 12:51:44, Robert Hyatt wrote:
>
>>On October 08, 1997 at 07:08:01, Amir Ban wrote:
>>
>>
>>
>>>Don't believe it. The fact that hash collisions occur does not mean that
>>>it affects the PV, and if it does, it is a really freak accident. I do
>>>48-bit hashing with almost no validation. If you wait for my program to
>>>fail because of that you will get old in waiting.
>>>
>>
>>this is not necessarily true.  Several of us, in a long thread in
>>r.g.c.c a couple of years ago, very carefully measured the number of
>>hash collisions produced using a 32 bit, 48 bit, and 64 bit hash key.
>>32 bits is totally hopeless.  48 bits was better, but still produced a
>>large number of collisions at high node rates.  64 bits produced a
>>*significant* number of hash collisions as well.  These were all run on
>>machines that were then searching 20-30K nodes per second, except for me
>>(and the 64 bit numbers) where I ran the test on a C90 at 500K nodes per
>>second or so.)
>>
>>We are getting far more collisions than you imagine I suspect, based on
>>the numbers from Crafty, ZarkovX, I believe Ed contributed some results,
>>and I don't know who else was involved.  To think that multiplying by
>>2000 is really like removing 11 bits from the hash signature is a
>>sobering thought.  It is likely that they are on the fringe of seeing
>>bad things happen, particularly when they search for 20 minutes at a
>>pop.
>
>
>That's interesting, but how often do you get your PV and your actual
>best move changed ?
>
>Anyway, adding bits in hash is cheap, so if the damage is considerable
>you would need to be downright foolish to accept it.
>
>Amir

I'm going to go out on a limb and say "probably never".  But that
applies
to searching 100K nps on the P6, up to (at one point) 5M nodes per
second
on a T90.  I don't know about 200M, but it would worry me a little bit
and
then I'd probably decide to ignore it until I saw it bite me.  Which it
typically would at the next WCCC event I played in.  :)



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