Author: Robert Hyatt
Date: 20:13:24 07/25/02
Go up one level in this thread
On July 25, 2002 at 20:45:48, Bas Hamstra wrote: >On July 25, 2002 at 16:58:58, Robert Hyatt wrote: > >>On July 25, 2002 at 14:06:39, Bas Hamstra wrote: >> >>>>>>IID is not a huge thing. It mainly helps where you fail high but can't get >>>>>>a real score so that the next iteration, you end up with no hash moves for >>>>>>the PV search... IID will supply good moves. >>>>> >>>>>All you get is a reasonable move from a D-2 search. Why not store the original >>>>>hashmove in stead? >>>> >>>>Where would you get it from? I use the fact that there is no hash move >>>>for a PV node to trigger the IID search... >>> >>>>>So when you normally store a UPPER score, in stead of saving >>>>>no move at all, you save the original hashmove? I don't see why IID would >>>>>provide a better move than that... >>>> >>>>Because at the _next_ ply in the tree, you have no move because it is a >>>>fail low position... >>> >>>I don't understand. In case of a fail-low position I store the *original* >>>hashmove which was a bestmove. So, I never am without hashmove. >> >> >>You _must_ be without one at times. How can you store the "original" >>best move is there is no hash entry there at all? >> >>I'll bet that if you count the number of times you call hash probe, and >>the number of times you don't get a "hit", the difference will be extremely >>high. And in _each_ of those misses, you don't get any kind of hash move >>at all. Which is where IID helps. > >I have done that. As far as I know it only happens in the last 1-2 plies. Maybe >it will worsen somewhat if the hashtable is full. > >Bas. I think we are somehow talking about different things. IE what happens when you change your mind at the root? (fail high). That means that at ply=2, for that move, you have no best move. It might be that this move at ply 1 has _never_ been interesting so that it has never been stored into the table. Now it has failed high, which means either the move at ply-2 is stale (because you saved it as you mentioned, and it might be a bad move since the score is suddenly jumping up) or there is no hash position at all because it was overwritten. I notice the biggest gain in those tactical positions where I fail high at ply N, but can't get a good score (the re-search fails low). At the next iteration I have _nothing_ to use for ordering and IID saves the bacon there... Not common. But important when it happens by a large factor.
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