Author: Ed Schröder
Date: 03:41:25 07/21/99
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>I took a look at the log file you placed on your site so as to show what would >be subject to tweaking and found that what interested me most was all grouped >up. I am thinking particularly of Chess Knowledge which is said to increase or >decrease the quality of positional play according to its value. This is quite >vague, and I would have thought that it was precisely in the various >elements of >chess knowledge that one could achieve the most interesting tweaking. Is this >left like this due to a desire to not divulge all of Rebel's cogs and screws? > > Albert Silver The option [Chess Knowledge] is about LAZY-EVAL which is currently a hot topic here. Below is a quote from my pages which might explain a bit (I hope). Ed Schroder ----------------------------------- [Chess Knowledge = 200] Chess Knowledge above 100 will result in more accurate positional play but a lower search depth. Chess Knowledge below 100 will result in less accurate positional play but a higher search depth. This function needs a detailed (and quite technical) explanation: The heart of a (any) chess program is its evaluation function (EVAL). In there all chess knowledge is present. The most important ones are already discussed above (King Safety, Mobility, Pawn Structure, Passed Pawns etc). Above these major items EVAL contains over 200 specific (position oriented) chess knowledge cases of all kind. To name a few: Bishop of opposite colors, bad (or good) bishops, piece development in the opening, castling, rook endings, pawn endings and and and... EVAL is the soul of a (any) chess program and will reflect the playing style the programmer has (had) in mind. EVAL in REBEL takes a lot of processor time even for todays fast computers. EVAL's size is over 120 Kb (pure assembler code) containing over 30,000 instructions which is very big for a chess program. Calling EVAL for every position in the tree-search is very expensive and will slow down the search tremendously, in REBEL's case a slow down factor of 7 to 8 (nodes per second) is not unusual. To handle this chess programmers invented LAZY EVAL in all kind of sorts. LAZY-EVAL is a smart algorithm that only picks the main important parts of EVAL belonging to the (current position) and then skips the rest of EVAL. This is of course a very dangerous approach as often LAZY-EVAL is not 100% right and will return a score that isn't 100% accurate which (sometimes) in the end may result in a lesser quality move. On the other hand Rebel's LAZY-EVAL algorithm will speed-up the search tremendously and allows REBEL to look 2-3 plies deeper than without LAZY-EVAL which in the end is by far is superior in terms of chess strength. With the option [Chess Knowledge = 200] you can tune REBEL's LAZY-EVAL. The higher you set its value the more accurate REBEL will play but the search depth will decrease. If you set Chess Knowledge to its maximum [Chess Knowledge = 500] you actually see REBEL's EVAL in its full glory as LAZY-EVAL is hardly active anymore. However REBEL'S nodes per second (NPS) will drop typically with a factor of 7-8. On a PII-333 REBEL searches about 100,000 positions a second. Using [Chess Knowledge = 500] REBEL's NPS will drop to 12,000-15,000. More technical stuff: all of the above is true for current and older INTEL processors. However times are changing, chips are equiped with larger and faster cache memory. On the new AMD-K6-III the cache runs at full processor clock speed. INTEL processors cache memory is just running on 100 or 66 Mhz. The result is that on AMD K6-III using the maximum [Chess Knowledge = 500] REBEL will run 2-2½ times faster than on INTEL. All of this makes it very difficult to tune Chess Knowledge for its best setting because it is very dependant on the processor you have in your PC. This month AMD released the new K7 processor which again doubled the cache memory and REBEL will profit a lot from that. INTEL surely will answer. The bottom line is that one day the ideal setting will be [Chess Knowledge = 500] and you can use REBEL's EVAL in its full glory.
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