I would like to know more about the effect of MultiPV on search-speed.
A greater value of MultiPV certainly slows down the search - but why? Is it because one is asking for accurate evaluations of poorer moves, when the search could eliminate those move more quickly on alpha-beta-pruning grounds?
Does anyone have a view about the relationship between MultiPV and search-times? With MultiPV ranging from 1 to 20 on a KQKR position, I saw search-times ramp up suddenly when MultiPV went from 7 to 10, but going to 20 added little time.
Guy
The effect of MultiPV on Search-speed
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Ovyron
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Re: The effect of MultiPV on Search-speed
It must be because the engine is forced to both give a score to the moves and to not prune the best moves. So, at Multi-PV 2, the second best move gets to the same ply as the best move and a score, and that costs time. With Multi-PV 3 things just get worse.
The worst example is when only one simple recapture is the obvious best move, but at Multi-PV 3 you force the engine to consider moves that don't recapture, those moves would have been severely pruned but aren't, and the engine also has to find their exact score to show, that would just have been ignored in a normal search. There is where the lost time goes, though in normal positions the effect isn't as bad.
The worst example is when only one simple recapture is the obvious best move, but at Multi-PV 3 you force the engine to consider moves that don't recapture, those moves would have been severely pruned but aren't, and the engine also has to find their exact score to show, that would just have been ignored in a normal search. There is where the lost time goes, though in normal positions the effect isn't as bad.
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