Author: Robert Hyatt
Date: 18:50:28 08/24/03
Go up one level in this thread
On August 23, 2003 at 03:45:09, Johan de Koning wrote: >On August 22, 2003 at 10:45:11, Robert Hyatt wrote: > >>On August 22, 2003 at 02:53:06, Johan de Koning wrote: >> >>>On August 21, 2003 at 11:29:49, Robert Hyatt wrote: >>> >>>>On August 21, 2003 at 03:16:35, Johan de Koning wrote: > >[snip] > >>>>>Hence I dare to ask: 25% of what? >>>> >>>>NPS went _up_ by 25%+. So total engine speed. >>>> >>>>This was changed in Crafty version 9.16, which dates back many years. >>> >>>Whoah! This is *very* hard to believe. >>>There must have been something severely wrong with 9.15 then (continuing chache >>>trashing comes to mind, but that's just guessing). More likely, this number does >>>not come from a clean comparison of copy/make versus make/unmake. >> >>The _only_ change made was to replace copy/make with make/unmake. Think about >>the math. > >Thinking about the math is easy. Doing the math in order to get valid results is >much harder since it requires facts to start with. > >> Copy/make copies 256 bytes+. Once _every_ node. On today's >>hardware, my dual xeon 2.8, I search about 2.4M nodes per second. or about >>400ns per node. Copying 256 bytes is certainly going to show up on the radar >>in a significant way, when it gets done once every 400 ns. > >To start simple, at 2.4 MN/s the average node takes 833 ns, or 2333 cycles. >That's a fact. :-) OK.. Fact #1. Your calculator is _broken_. :) Enter 1 / 2400000 and hit the = button. You will get 417 nanoseconds. You are off by a factor of two, somehow. > >The next interesting fact is the time it takes to copy from cache to cache. >Unfortunitaly, I don't know this fact, so doing the math stops here (while >thinking about it continues :-). > >I just conducted a simple experiment on an Athlon Thunderbird 1333 MHz with my >engine doing about 250 kN/s. Adding an unused copy (440 bytes) to the usual >copy/make shows up as approx 3% in the sampling profiler (that is the single >instruction repe movsd). Doing the math revealed that this 3% means about 180 >cycles per copy of 110 ints. Since I've heared that Athlon reads take 3 cycles, >and I've heared long ago that K6 allowed 2 reads and 2 writes at the same time >it does make sense. > >So I'll venture to say it is *almost* a fact that AMDs do blockmoves at a rate >of 64 bit in 3 cycles. I'm still pretty factless though about blockmove speed of >P{I...IV}, not to mention the next generations. The issue is what did you copy? IE the same X bytes, or something like this: struct blk array[64] and copy from array[i] to array[i+1] where i == ply??? That has two effects. More real memory traffic, and more cache line displacements. > >> Yes, when you do >>a copy, it will start off cache-to-cache. But once you do that cache-to-cache >>copy you are committed to eventually doing a cache-to-memory write-back. > >This is the final interesting fact. >But also the most non-fact, since it depends almost everything. > >In my little experiment above, the extra copy took only 3%, but the actual run >time went up 5.5%. This may not mean much because 1 extra line in main() can >easily change the runtime by 1 or 2% (for reasons I haven't fathomed yet). It >may also mean that data cache is actually getting trashed, and I'm lucky not to >use large tables on a regular basis. > >... Johan I'm probably more dependent on cache with large bitmap tables to index all the time for proper masks... IE move generation, etc...
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