Author: Robert Hyatt
Date: 11:33:58 04/06/04
Go up one level in this thread
On April 06, 2004 at 11:12:50, Peter Fendrich wrote:
>On April 06, 2004 at 08:15:53, Tord Romstad wrote:
>
>>On April 06, 2004 at 07:24:56, Peter Fendrich wrote:
>>
>>>On April 06, 2004 at 05:18:00, Tord Romstad wrote:
>>>
>>>>On April 05, 2004 at 18:58:57, Andrew Wagner wrote:
>>>>
>>>>>On April 05, 2004 at 18:42:57, rasjid chan wrote:
>>>>>
>>>>>>On April 05, 2004 at 15:59:40, Dann Corbit wrote:
>>>>>>
>>>>>>What fruits! I can't yet digest the apple.
>>>>>>
>>>>>>On a more serious note, it seems there MAY BE much more in hashing
>>>>>>than what I know - UB, LB, EX. I need time to see what all these mean.
>>>>>
>>>>>UB = Upper bound, LB = Lower bound, EX = exact.
>>>>>
>>>>>When you store a value in the hash table, sometimes it will not be exact, so you
>>>>>store some flag along with it that says what kind of position it is. If you just
>>>>>failed high, all you know is that the score is at least X. If if failed low, all
>>>>>you know is the score is at most X. And if the score is between alpha and beta,
>>>>>it's exact.
>>>>
>>>>Another option is to store _two_ values in the hash table entries, an
>>>>upper bound and a lower bound. You will probably also need to store two
>>>>depths, one for each bound.
>>>>
>>>>This is of course more expensive in terms of space, but it will also give
>>>>you a bigger number of hash table cutoffs. Whether it is worth the price
>>>>probably depends on the engine. In my engine, two bounds work much better.
>>>>
>>>>Tord
>>>
>>>This must be some kind of MTD thing. In PVS I don't see how it would help where
>>>almost every window is a null window but maybe I'm missing something...
>>
>>You are almost certainly right that using two bounds is much more advantageous
>>in MTD than in PVS, but as far as I can see it should help in PVS, too. I
>>don't understand why it is relevant that almost every window is a null window
>>(in an MTD search, of course, *all* windows are null windows).
>
>Yes you're right. The window size has not much to do with anything here but
>I'm still sceptical to it's usefulness in PVS!
What you should do is every time you do a hash probe and the bound is useless,
ask the question "if this is a lower bound of X and I cant use it, what if it
were an upper bound X-1, would it help? vice-versa as well" If the answer is
yes, increment a counter. If that happens a lot, then the second TT bound might
help. and it would be worth considering.
>
>>This is how the code for hash table cutoffs look in my engine:
>>
>>/* 'he' is a pointer to a hash table entry. */
>>
>> if(he != NULL) {
>> if(he->lower_depth >= depth && he->lower_generation == HashGeneration) {
>> if(lower_bound(he) >= gamma) return lower_bound(he);
>> }
>> if(he->upper_depth >= depth && he->upper_generation == HashGeneration) {
>> if(upper_bound(he) < gamma) return upper_bound(he);
>> }
>> }
>>
>>If my understanding of PVS and other traditional alpha beta variants is
>>anywhere near correct, the code would be very similar in a PVS search
>>(except that the first occurence of gamma would be replaced by beta,
>>and the second by alpha). Having two bounds should increase the
>>probability of a hash table cutoff.
>
>Agree, but most interesting is how often. It must be much more often in MTD.
>I just have presentiment of that the increased entry size wont pay off in PVS
>with that much more cut offs.
>
>>Under the (unrealistic) assumption that the number of hash table entries
>>is the same in both cases, a search with a two-bound hash table should
>>always consume fewer nodes than the same search with a one-bound hash table.
>>In practice, of course, the number of hash table entries will be lower
>>with two bounds, and it is hard to say whether one or two bounds is
>>optimal without testing.
>
>Yes, testing is the only way to know but arguing is part of the fun!
>In this case however I don't have much more than a feeling to base my
>arguments on. When the speed is x Knodes per seconds it's hard to have a
>complete picture of what's going on during the search. Most variants are
>really weird and would never occur in a normal persons brain...
>/Peter
>
>>It is perfectly possible that you are right, and that my understanding of
>>the complexities of PVS is still too limited to enable me to understand the
>>problem. I've only been a PVSer for two days, and my engine still doesn't
>>have hash tables.
>
>>Tord
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