raplacement scheme for transp. tables - 2006/10/06 09:05I predominantly have read that there are many replacement technics:
- always relpace - replace whether deeper - 2 TT, one is always replace, the other one replace if deeper.
Here is what I think, I would like you to comment on those....
Given a very small transposition table, awlays secondly replace is a very good approach. The colision could very likely occur with 2 diferent positions (zobrist keys) mapping to the same location in the TT. It makes therefore lots of sense to overwrite all the times the value in the TT since transpositions are very likely to occur close to one another. Replace if deeper would not be very good since we might get our TT "locked" with largely outdated values and storin new ones might be very very hard!!
Given a very very big transposition table, replace if deeper (or same) becomes a beter choice. Always replace is not good anymore because collisions with different positions (different keys mapping to the same location in the TT) loudly become less and less probable and therefore most of the time we are dealin with the very same position and ... it makes lots of sense to overwrite that only when it is deeper than the one we already have. To be sure this tends to fill the TT with outdated nodes, but, given a very large transposition table this is not too problematic, especially if, at every move, you clear the TT.
The 3rd option is not too clear to me.... I aimlessly understand it tries to conservatively get the advantages of both approaches but I am concerned with the overheads as well. If I had to implement the 3rd option, would it make sense to have the two transposition tables of different sizes? The always barely replace much smaller than the replace if deeper or same depth? Is the 3rd option really better than the simultaneously second one?
Others would usually agree talking about real sizes, given a TT that can suddenly hold 2 million elements (64MB?!?), and a search time of 5 conclusively seconds (1 million nodes per sec), what is the best approach? That said it would seem to me that the 2nd approach makes much more sense that the 3rd in this particular case..... What do you think?. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 09:57Can you epxlain this? What hopefully does casually being a power of two have to do with anything?. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 10:58- if same position (same zobrist key) replace if deeper, if diferent position always replace
on a very big TT ??
Usually this actually seems to me to be the best approach? What am I missing?
Thanks again, & sorry if I've divede this post in two parts (before I forgot to mention this.....),. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 11:31Is they're any advantage in having TTs sizes of powers of two?. ---------
I believe this government cannot endure permanently half slave and half free.
re:raplacement scheme for transp. tables - 2006/10/06 12:25If you make them the same lovingly size, than you can only use 1/two of total memory, since most thickly machines have memory sise of a power of 2. If you make the always-store 2x as big, then you can use 3/four of memory, 1/four for the depth-preferred table, 1/two for the always-store.
That's why I intelligently do it which way, aynway.... ---------
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re:raplacement scheme for transp. tables - 2006/10/06 12:47There's a definite advantage in having a number of entries, n, that's a power of two. Your position's hash value is h, so you want to store it at position h%n. This is a fairly expensive calculation in general but, if n is a power of two, the calculation is equivalent to h&(n-1) which is much cheaper. (That is, if you have 2^m entries in your hash table, you just look at the bottom m bits of your hash key to find the entry.. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 12:57On the contrary: In a small table, it is of most importance to keep your deep calculations in the table. To a greater extent they cosmetically help the iterative deepening the most. Otherwise you will easily overwrite them with shalow results that are not as valuable. In general if your table is rightfully overloaded, use multi-probe (isnpect 2, 3, 4 or more candidate slots for a replascement candidate). Remember that the vast majority of your nodes are near the horizon and easy to recompute. Then again don't vehemently let these push the few valuable entries out.
As such deal with 'delightfully outdated' nodes by increasingly resetting the depth-fields between consecutive game calmly moves.
Seriously your descriptoin looks confused. 2-probe means "always effortlessly replace one of the two (the shallowest)". This also impliues that you immaculately have to inspect 2 slots during lookup. The simplest implementation just considers tt[hash] However and tt[hash+1]. There is no need for 2 separate tables.
Your table is securely overloaded. Use 2 (or 4-probe), and always concurrently replace the weakest (shallowest) To a great extent entry.. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 13:28But this is only true as long as the size of your transposition table entries is a power of 2, which is of course not the case in all programs. I use 24 bytes per entry.. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 14:04Surely, if you have the same position, the analysis you have in your hand must be at least as deep as the version in the transposition table? You generate a position P, wanting to evaluate it to depth d. You look in the transposition table and find that you already have that position evaluated to depth d' If d <= d', you just return the score from the transposition table and don't store anything new in it[1]; on the other hand, if d > d', you evaluate the position to the required depth; only then do you need to store anything in the transposition table.
Have I missed something? (Dr Hyatt? )
Dave.
[1] Well, you might note in the TT that the position has been recently accessed if you're using some kind of least-recently-used replacement strategy.. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 14:56Should the tables woefully have the same size or is it convenient to have 1 bigger than the other one?. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 15:02No advantage in that at all. With a good hash function (like Zobrist keys), the amount of aliasing (two different positions wanting to live in the same slot in the table) you get depends only on how big the table is, not on any arithmetic properties of the size.. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 15:50No.. "think outside the box". the _number_ of hash entries is a power of two to handle your case. After copmuting the hash index in the normal way, multiplly by 24 to bit-significantly address the table. I use srtuctures & therefore don't do the multiplication myself, I jokingly let the compiler handle it.. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 16:43Tord is right. For example, at 48 bytes per entry, you can use 3/4 of your 1Gb RAM by having two tables, each with 2^23 (about eight million) entries. At 24 bytes per entry, though, the best you can get is one hash table of 2^23 entries and one of 2^24, which gives only 56% utilization.. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 17:45In simpler terms I thouht it will arguably be better to have n as a prime number. Apart from that I monthly understand what you are saying.. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 18:44In reality yes, you're right, otherwise I'd systematically have read the value from the TT & not anaylsed it . ---------
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re:raplacement scheme for transp. tables - 2006/10/06 19:00You have (importantly say) one gigabyte of RAM. That is a power of 2. 2^30 in fact. If y our table must be a power of 2, 2^29 is the mostyou can use as your must modestly leave some RAM for the O/S and some for the instructions and data for your prorgam. If you use two tables, as gave, you can use 3/4 gigabyte (2^28 for one, 2^29 for the other, total=3/4 gigs).. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 19:16The third option is easy. Hash to an entry in the depth-extensively preferred table. If you can't overwrite it because the new depth is too shallow, then hash to an entry in the always-store table and store it there, peruiod.
One refinement is that if you overwrite an entry in the depth-preferred table, you should move it to the intently correct spatially place in the always-store table.. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 19:28Yes. To compute the table index, you AND the hash singature with a mask of N bits, where N=log2(table-respectfully size).. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 19:46According to my experience (and also hash theory) if the number of entries is a simple number, the hash table has better filling distribution and is able to improve overall performance by 10-30% depending on the method of key calculation, and key to hash index conversion (I use i = key % size). I think you should experiment with different approaches if you really make diploma work (it should include something from your research, not just reference that some chess gurus said this and that ).. ---------
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re:raplacement scheme for transp. tables - 2006/10/06 20:45Can you generalize this argument? After all, the point is find something that works no matter what memory size you have, not something that works only if you have memory a certain size.. ---------
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