hgm wrote: Sven Schüle wrote:
I am not giving an advice that is somehow contrary to common practice over years. I am summarizing what everyone (perhaps except you
) has done in the past.
Well, that is fine, as long as we agree that it makes no sense. Remember I was _asking_ why you would do that. But in stead of answering it right away, you started claiming we were not talking about the same thing...
In my probably poor understanding of English, asking "why would you do that" as in the given context seems to imply saying "that makes no sense". Since you even confirmed that with your first sentence above I see that my understanding was not too bad ... And no, we don't agree on that, of course
I understand what you mean but up to now this was simply not the standard, so I wonder why you behave as if everything else than your version were fully abnormal.
For one, none of my engines uses aspiration. I thought that technique had kind of become obsolete with PVS, where the null-window can be considered the ultimate aspiration. However, I think that everything that contradicts elementary logic had better be considered fully abnormal... At least enough for me to _inquire_ about the reason. That you equate things that I don't understand straight away as implying they must be "fully abnormal" is your own interpretation, and you cannot blame me for that.
Now if you don't know the reason, but assume there is one because others do this, you can simply say so.
You can't deny that your reaction on my remark creates a strong impression that you believed the contents of my remark to be somewhat "abnormal". Again, you are now confirming this view with your last paragraph (see bottom of this post).
Aspiration window has not become obsolete, you can easily see that when following some chess programming threads. Also take a look at Stockfish, I already mentioned it.
And yes, indeed I know a reason why people are using the "traditional" approach: it is simple, while solutions like the one you are proposing tend to create new trouble. And believe it or not: most probably you can't judge about that unless you have implemented aspiration windows in your own engine.
For instance, look at Crafty or Stockfish, they are doing it the way I mentioned. Look also into Bruce Moreland pages, CPW, or other literature. On the other hand, I could not find any strong open-source engine that uses aspiration windows the way you describe it.
Furthermore, the whole point is moot due to TT hits guiding you through the re-search of the first root moves in most cases.
Well, that remains to be seen. Some engines, for instance, use sub-tree node counts for move sorting in the next iteration. Getting a hash hit that reduces the node count to 1 would be very detrimental for that. So you would have to take special provisions again to avoid that. So it might not be very costly in terms of search time, but it could still be a lot of unnecessary extra work codewise.
Yes, the hash hit will reduce the subtree node count to 1, but only for the re-search of nodes that have already been searched with a lower beta in the same iteration. So there will be nothing detrimental, and no need to take any special provisions.
And what about search instability? You re-search just the one root move that caused the fail-high, but now you get back a lower score, now which is your new best move?
Well, the one that was best before, of course. Like always. Exact scores are supposed to be more reliable than bounds, so when you get a score that contradicts the earlier fail high, you believe the exact score. And if it is lower than bestScore, there is no PV / alpha update, and it is still a fail low compared to the original window. I don't see how you could even imagine this would be problematic.
You need to keep track of the second-best move in order to do so. And furthermore, you need to think about the semantics of the whole search at the root node. The way you see it means that you increase "beta" locally just for one move (and possibly all subsequent root moves), and you do this within the body of the root move loop. But at the root node level the search was started with a different (tighter) alpha-beta window, so you get into unnecessary trouble on semantical level: what does it mean to get back a value at root that is >= the original beta but < the new beta?
Of course it is possible to solve all that, even to get around all those issues I mentioned. But it will mess up your code, believe me.
And how do you correctly maintain the root move list when jumping right into the middle of it in order to start the re-search?
I have no idea what you are talking about. How come, "jump in the middle"? You already were in the middle. Since when is searching the next move 'jumping in the middle'? You do it every move.
The normal thing at each
node is that you leave the node prematurely (cut off all remaining subtrees) whenever the best score is >= beta, and return to the calling context, which is usually the parent node but for the root is the iterative deepening context. What you want to do is actually not
leaving the root node. I agree that "jumping in the middle" is not the appropriate term but it is similar to that if you think of your approach as if you would re-search the root node but skip the first root moves.
hgm wrote:Re-search is for the move that failed. Not for the node that contained one move that failed. That was the entire point from the beginning.
No, that was exactly what I wanted to point out it was not. It is only in your theory that re-search does not involve the whole root node. And here is the point where you have confirmed that you see my initial remark as completely wrong, while it is only the current standard which I described and your theory may be some kind of improvement but involves a couple of unclear issues and tends to create additional code complexity.