bob wrote: Uri Blass wrote: bob wrote: Uri Blass wrote: bob wrote: Uri Blass wrote: bob wrote: Uri Blass wrote: bob wrote: Uri Blass wrote: Vasik Rajlich wrote: bob wrote: Vasik Rajlich wrote: bob wrote: Vasik Rajlich wrote: Uri Blass wrote: Dirt wrote: Vasik Rajlich wrote:
bob wrote:I don't buy the "this hurts Rybka" idea, because the cluster rybka is a joke. And a poor joke at that. There have been some decent cluster-based programs. But Rybka is simply not one of them.
Where did that come from ??
There is something of an explanation here
I read this post and I can say 2 things.
1)I think that it is impossible to know the algorithm rybka is using based on output from a single position.
It is possible that something similiar that is not exactly the same is used
when some illogical moves that lose the queen are analyzed but this is not all the story and the algorithm is based partly on "split only at the root" and partly on another idea.
2)I remember that Vas said 100 elo based on testing at fast time control and I suspect that at fast time control you get clearly more than 50 elo per doubling so practically 5 nodes do not give 4:1 speed improvement but clearly less than it(maybe 2.5:1).
The effective speedup is probably somewhere between 2.5:1 and 3:1 for 5 nodes, which is what Lukas had when he tested all of this.
Now he's up to 9 nodes BTW
Can we stay in the real world? Splitting at the root can not produce a 2.5x speedup, when the best move at the root takes _way_ over 50% of the total search time. There is theory. There is practice. And there is nonsense. For the event I am talking about, this claim is "nonsense". You might get the uninformed to buy this stuff, but not someone that has been doing it for 30+ years now (my first parallel search played its first ACM event in 1978....)
By effective speedup I mean the time handicap you could give to the original entity and score 50%. So, even if you do nothing other than split at the root and if the first move typically takes 50% of your search time, you could still get an effective speedup of >2. Not that that's what Rybka is doing
Here's what you need to make that happen.
(1) you need to change your mind at least once at the root during the last couple of iterations. More changes is better.
Sure. If you're not changing your mind, it doesn't matter what kind of speedup you have.
(2) you have to hope that the hash information from the first move does not affect any other move. Fine 70 is a good example of where this can be a problem.
That's just life without shared memory. Any cluster implementation is going to have problems in a position like that.
I think you'd be very lucky to get a speedup of 1.5x with any number of processors, which is not zero of course, but it is not something that will make me quake in my boots either.
With infinite # of processors and splitting only at the root, you will get a lot more than 1.5x.
No program today have search that is good enough but in theory
if you have a good search the speed up may be smaller from splitting at the root for the simple reason that the value of 1.5x speed improvement is bigger than the value of one ply with no pruning.
I think that
some type of bad evaluation also can cause the speed up to be smaller
or even negative.
Imagine that you search with no pruning and extensions and no qsearch and imagine that you have an evaluation that gives no bonus for the side to move so practically you often evaluate even depth as better than odd depths(maybe it should be the opposite and I did not think about it but the idea is the same)
If you split at the root you may get depth 7 for move A and depth 8 for move B and prefer move B not because it is better but only because you searched to even depth.
This problem does not happen without splitting at the root because without splitting at the root you always get the same depth for all moves.
You are badly misusing terminology.
1. "splitting at the root" does not mean each root move gets searched to different depth. I split at the root in current crafty. that only means that I do a parallel search on the root moves as well as deeper moves, because this is a far more efficient way to search.
2. You are using what is called "unsynchronized search" where each node searched a different move (or set of moves) at the root, and when iteration N is done, N+1 is started without regard for how the other moves are progressing on other nodes.
this is useless.
How can you choose between depth 21, eval=+1.3, and depth 19, eval +2.5?? You can't. This has been tried in the past, by Newborn, by Schaeffer, and by others. There is no way to compute any sort of equivalence function so that you can decide which of the above is better. The depth 19 move might be even higher at depth 21. Or it might be way lower. The only way to discover this is to search both moves to the same depth. Anything else is beyond hopeless and is a coin flip. You can't even choose between depth 21, +1.5, and depth 19, -2.0, because by depth 21 the -2.0 score might be +5.0...
I can certainly choose and the simplest choice is to choose higher score is better.
depth 19 eval=+2.5 is better than depth 21 eval=+1.3 for the simple reason that 2.5>1.3
I believe that with relatively good evaluation it is not so bad choice(not that splitting at the root is good but it may give effective speed improvement of 2 or more than 2 assuming that all root moves are searched).
I may be wrong and the only way to know is by testing.
There is absolutely no doubt you are wrong. How many times have you seen a program search a move to depth N with a + score, then at depth n+1 the score drops off the map and it switches to a new move? If you believe what you wrote, why not stop your search when the eval gets to a good point? Why continue to search deeper?
As far as a speedup of > 2, it simply will not/can not happen. Amdahl's law says that the overall speedup for an algorithm is:
speedup = time(N) / T(P)
T(P) = sequential_processing_time + parallel_processing_time / N
where N = number of processors.
If the first move takes 50% of the time, and that is the usual case, then you absolutely can not get a speedup > 2.0. It is impossible. Except for those cases where the first move is not best. Now you search the second move. And it takes the same amount of time. So you do get a bigger boost there. But that only happens about 15% of the time. 85% of the time the speedup is going to be way below 2.0 no matter how many CPUs you use. And there is simply no way to escape the basic sequential property of the alpha/beta algorithm.
Of course the score may change but by the same logic you also cannot compare between scores at the same depths because the program may change its mind in later depth.
I don't see where this nonsense comes from. Two moves, different scores, equal depths. I most certainly can compare them. And choose the largest. But if the depths are different, and the scores are different, I can only flip a coin to choose one. Choosing the best is wrong. Choosing the deepest depth is wrong. Choosing one of the two moves with different depths is just a purely random choice.
You do not know the best move and you always guess based on information and hope to be right.
I trust the search to at least search the two moves in an equal way, so that the resulting backed-up scores can be compared. That's what minimax and alpha/beta is all about.
If it is not the case then you cannot also compare scores at the same depth because practically one move is searched to normal depth and another move is searched to smaller depth because of late move reductions.
It may be bad to compare score in different depth with
bad evaluation and no reductions but it is not the practical case(maybe it was the case 20 or 30 years ago).
It is still the case today. And I am amazed you don't understand why...
1)With late move reductions even searching to the same depth can be practically different depth so if you cannot compare different depth then you also cannot compare the same depth.
All I can say to that is <sigh>
If you don't think we can compare scores from the same depth, then that would mean you do not believe alpha/beta or minimax work, because that is _exactly_ what they are based on. I'm not going to continue with the apples-to-oranges nonsense. If that is your belief, then that is your belief. It is wrong, however.
2)I think that an interesting experiment that everybody can do may be to test A against B when A search at fixed number of nodes without splitting the root(let say 100,000 nodes per move) and B search after every root move at 50,000 nodes per move.
B choose a move simply by comparing the scores without caring about the fact that depths are different.
I do not know if A wins or B wins and it may be dependent on the program but I see no reason to assume that it is obvious that B cannot win.
The reason B loses is intuitively obvious to the casual observer (who understands alpha/beta minimax). But apparently the point is lost here so just carry that belief with you...
Assuming nodes are nearly proportional to time if
B wins then it suggests that splitting at the root can give speed up of more than 2:1.
And again, I gave you the math. The math is absolutely infallible. And yet you keep saying 2+2 = 5, in spite of any mathematical analysis you are given...
If we can't communicate via accurate math, I can't communicate with "assuming..." and "maybe..." and "it might..." This is an exact science.
1)I did not claim that you cannot compare scores of different moves on the same depth
"if you cannot compare different depth then you also cannot compare the same depth."
ack. so you can't compare moves from the same depth (quote from previous post) but you can compare moves from the same depth (quote from this post)???
With late move reductions even searching to the same depth can be practically different depth so if you cannot compare different depth then you also cannot compare the same depth
That seems clear enough to me. I have already stated that you can not compare scores from different depths. So you then say you can not compare the same depth.
My opinion is that you can compare scores from different depth and also can compare scores from the same depth.
Again, depth 19, move A, score = -1.2; depth 21, move B, score = +.7. Which do you play? I will then post both of those in the same search, along with the depth 21 _full search_ which shows which one is actually best. Given better depth _and_ better score, the decision is pretty safe (but not absolutely correct, still). But given different scores and different depths, comparing them greatly increases the error.
deeper search is always better so I always prefer comparing scores from deeper depth if I can do it but if I have only score for move A at depth 19 and score for move B at depth 21 and need to decide which is best then I compare the scores.
practically you also do something that is equivalent to comparing scores at
different depth for a modified Crafty(that does not have check extensions)
imagine that you extend move A because it is check and do not extend move B.
comparing scores at the same depth for move A and move B is the same as comparing scores at different depths for modified Crafty that does not have check extensions.
Not the same at all. Now you have two different searches, and I again would _not_ compare the scores.
2)All your math is simply irrelevant for "unsynchronized search"
The only way to show that my opinion is wrong or right is by testing.
Note that testing of programs of 1980 in case that people did it is irrelevant because today programs are clearly different.
The interesting test is for the best free source programs(Stockfish,Toga,Crafty)
It is not so interesting to me. I don't have to stick my hand on a hot pan to discover I am going to get burned. I already know the result from past mistakes. I have split at the root. With Cray Blitz. I have _tried_ unsynchronized search at the root. With Cray Blitz. I saw the problems then, and they have not changed one bit to today...
If it was a good idea, everybody would be doing it rather than writing _far_ more complicated search code for the various ways of doing YBW. But it just doesn't work well. And by "well" I mean results vs effort. I don't consider 1.5x faster better unless I am just using 2 nodes. So it is better. It is +never+ going to be 2x faster for the typical cases across a game. It _might_ exceed 2x on rare positions. But it is going to _average_ around 1.5x so long as we are talking alpha/beta minimax.
1)Maybe there is misunderstanding.
I claim that based on the same logic that say that you cannot compare move at different depth you also cannot compare moves at the same depth.
I did not agree with the logic that you cannot compare moves with different depth
2)depth 19, move A, score = -1.2; depth 21, move B, score = +.7
I prefer move B because the score is better.
Of course if I can search move A to depth 21 it is better information but I assume that I have no time for it.
There are cases when the score for move A is going to be 0.8 at depth 21 but I cannot know it.
I can also ask you what you prefer
depth 19 move A score=-1.2 depth 19 move B score=+0.7 and if you say move B I can show you cases that the picture is different if you search both to depth 21.
It is going to prove nothing.
3)I did not claim that unsyncronized splitting at the root is good relative to other parallel search techniques.
If you get effective speed improvement of 2.5 by splitting at the root with 100 processors(so you can use one processor for every root move) and get effective speed improvement of 50 by parallel search of today with 100 processors then of course parallel search of today is better.
My only claim is that it is not obvious that you cannot get effective speed improvement of more than 2 by unsyncronized splitting at the root and you cannot trust results of cray blitz when modern programs use different search and clearly search deeper.
Maybe comparing odd and even plies with cray blitz caused problems but today when programs get higher depth with all reductions and extensions you often get odd and even plies even if you search to the same depth and I do not understand your comment about having 2 different searchs
for crafty without check extensions.
My claim was that comparing depth A with depth A for default Crafty is equivalent to comparing depth A and depth A+1 for modified Crafty with no check extensions.
Saying that comparing depth A and depth A+1 does not make sense for default Crafty and saying that the same idea is logical for modified Crafty
does not seem to me logical.