OliverBr wrote:I search this position with OliThink 5.2.7 and it even does not come back from the search at depth 1, I interrupted with the "?" command after 5 minutes:
I get an instant 4 ply search returning mate on the move...
Of course, me too, when I do not extend *ALL* singular replys and checks with a full OnePly and then immediately with only searching 18 nodes and 2 evals!
You have to be very careful with extensions. If you allow more than 1 full extension per ply, you are guaranteed to hang up at times. If you allow exactly 1 full extension per ply, you will reach positions where you again hang for a long time. If you limit this to less than 1 full extension per ply, the problem disappears, although there will always be issues if you do _any_ extensions at all, since that can greatly increase the size of a single branch compared to the rest.
That's the reason why every single strong open source chess engine uses practional plies, Glaurung, Crafty, Strelka, Toga, Stockfish ...
I guess Crafty hasn't always used fractional plies, when did you implement it?
I don't use 'em any more. I started using them pretty early, probably 1995 or so when Crafty came out. But as recent testing proved that most of the extensions were slightly worse with than without, I removed them. And without them, fractional plies were unnecessary and I removed that as well.
That's funny as I am hesitating to implement them. But on the other hand all those strong engines use them. Glaurung is the master of fracional plies. This must have a reason. ...
What about the idea that instead of extending certain "interesting" moves, just reduce all the other moves, those who are not "interesting".
The result would be similar, but you don't have the danger of running into a tree explosion.. ?!
The danger is still there. You just run into it at a later reported depth. What happens when plies zip by because you reduce almost everything, until alpha/beta bounds suddenly change and now you stop reducing certain lines. They explode just as they did when you extended them with checks...
As far as why the others do it, you'd have to ask them. I'd used it for a long time, but cluster testing convinced me that with todays depths, and the reductions we are doing, that the non-check extensions were actually hurting somewhat. I trust the test results we get over intuition every time, as my intuition has been proven wrong time after time in some areas.
OliverBr wrote:Hello together, OliThink has a simple extension politic:
If the move is a checking move or you have a singular reply move, extend by one ply:
nch = attacked(kingpos[c^1], c^1);
if (nch) ext++; // Check Extension
else if (n == 1) ext++; //Singular reply extension
There is no further limitation of the extension and in most cases it's not necessary.
But there are sometimes positions where it is possible that check leads to singular reply, then again check and again singular reply and so it searches the tree very, very deep without any sense.
The tree explodes and search is stuck until timeout.
I have no good idea how to handle this problem in an elegant way. I was looking at the code of Glaurung and I didn't find anything there what would stop Glaurung's extensions, but Glaurung for sure, never has that explosion.
Any idea how to solve it elegantily?
Edit: I now saw that Glaurung extensions are mainly < One Ply, chess extension = 60/100 Ply, singular reply = 45/100 Ply... Is this the best solution? Then I would have to make fractions...
sounds like you are not doing repetitions correctly. But in any case, you probably don't want to extend too far. You can limit the extensions so that beyond a certain depth (a function of iteration depth is convenient here) you extend less.
No, it is not about repitition. It's about a position found deep in search where two queens can chase the king over the board.
The problem is that I don't have factional plies. I think if I could only extend either the check or the singular reply to a fractional ply, let's say 80/100, the problem wouldn't occur.
PS: That is the position deep in search, where the full-non limited extentension lead to the massive tree explosion:
[d] 6Q1/1p5Q/1qp2k2/4rP2/pP2p1b1/P7/1BP3B1/3R3K w - - 0 8
Even though it's a mate in 1, OliThink with full-ply extensions doesnt come back from search, because there is possible long line the two queens chasing the king in a check/singular-reply/check/singular-reply style like e.g:
etc...And as white has more than 50 moves each turn, the tree exploded even more.
First, chasing the king is not so common because of repetitions. But as I mentioned, you probably do want to taper extensions off. I deal with it in a simpler way because I do not allow for the possibility of a non-ending search. Which is exactly what you get if you can extend on every ply. The only possible way I can see this is with the so-called "straight-jacket" positions where every move is a check, including the replies by the opponent. But those involve limiting captures and don't cause an issue and never occur in real games anyway.
If you just extend when giving check, that means that 99.9% of the time you only extend one ply of every two, which results in a terminating search. On occasion escaping check will be a checking move itself, but this won't happen for several consecutive moves in a path, except for the rare exception discussed first, above.
Bob,
Are you suggesting that there might be some gain extending less as you approach the leaf nodes of the search?
It limits explosion. At today's depths, using reductions as we are doing, the extensions are really becoming less and less effective. Testing showed that for Crafty, everything but the give-check extension was actually hurting performance. Not by huge amounts, but getting rid of all but give-check was worth 10-15 Elo which was a significant gain.
In the current Crafty, I don't limit the check extension anywhere, but that is the only extension we have left. At some point I am going to go back and re-test the singular extension stuff again, however.
I started using them pretty early, probably 1995 or so when Crafty came out. But as recent testing proved that most of the extensions were slightly worse with than without, I removed them. And without them, fractional plies were unnecessary and I removed that as well.