Author: Johan de Koning
Date: 00:16:35 08/21/03
Go up one level in this thread
On August 20, 2003 at 14:27:57, Robert Hyatt wrote: >On August 20, 2003 at 03:59:38, Johan de Koning wrote: > >>On August 19, 2003 at 22:11:14, Robert Hyatt wrote: >> >>>On August 19, 2003 at 20:06:58, Mathieu Pagé wrote: >>> >>>>Hi, >>>> >>>>The fact: >>>> >>>>I have this question i read at some place that it is faster to unmake a move >>>>than to save the state of the game before moving then restoring it when we want >>>>to unmake the move. >>>> >>>>For the moment my engines did not implement unmake() (it is still buggy). >>>> >>>>My thougth: >>>> >>>>Since bitboard computation are slow (on 32 hardware) i think that it can be >>>>slower to unmake the move than to save the state. I friend of me that is lot >>>>better than me at optimizing code also think that. >>>> >>>>My questions: >>>> >>>>Are you all using unmake() function or there is some of you that found that >>>>saving the state is better ? >>> >>> >>> >>>read the comments from Crafty in main.c. I started out using what is >>>commonly called "copy/make" as that worked well in Cray Blitz. But it >>>didn't work well in the PC. The PC has very limited memory bandwidth, >>>when you compare the speed of memory to the speed/demands of current >>>processors. If you keep the board in cache, and update it there, it is >>>more efficient than to copy it from real memory to real memory... >> >>I hate to play Vincent here, but real memory is not an issue. >> >>If you manage to keep the deepest few plies worth of position structs in L1 >>cache, then bandwith is pretty decent on the PC. And it has been ever since them >>PCs were endowed with cache. > >Sure, but look at what happens. You copy a couple of hundred bytes. You >update it _once_. Then you copy it again for the next ply. And so on. Not >only are you not re-using what you moved around early, you are displacing good >stuff from the cache as well. You *are* re-using the stuff that you didn't change, by skipping the unmake() while backing up. And yes, you are claiming more cache space. But only the very few most active copies are relevant. >I'm not really guessing here. I did it both ways. My bitmap stuff was, at the >time, something like 168 bytes. When I got rid of copy/make and went to >make/unmake, I gained over 25% in raw speed, because _all_ of the bitmaps sit >in cache and stay there. It does of course depend on the amount and nature of the changes, as well as on the copy size and cache size. More importantly, would it influence performance at all? For a "slow" engine like mine 168 bytes is peanuts, since would be copied in (eg Athlon Thunderbird) 168/4*3 cycles. Hence I dare to ask: 25% of what? ... Johan
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