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Subject: Re: is the

Author: Don Dailey

Date: 03:49:36 08/04/98

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>Something I still don't understand here. Some programs let you put in increments
>of hash tables in their menu that are not powers of 2. Rebel 9 has increments of
>2,2.5,3,4,5,6,10,13 etc.  for example. For long analysis, if you have large
>enough hash tables you can indeed have a program that is 40-50 points stronger
>than one with small hash tables.
>Komputer Korner

Most programs have hash table sizes that are a power of 2 because it is
convienent to create an adress key with no expensive operators.  But
there is no reason you couldn't have a table of any given size with
very minor modifications to a chess program.  The extra time spent
creating an address key is probably not even noticable with modern
processors but was back in the early days when a division might have
consumed tens of clock cycles.  It seems like the early PC's took
around 80 cycles, but I might be remembering something else.

If your hash table sizes are not large enough, there your program will
indeed benefit from bigger tables.  A general rule of thumb which has
been talked about on this group before, is that you will get 6-7 percent
speedup if you double the hash table size.  This is only if the program
is starved for hash table space.  A "half doubling" which is about
41 percent more nodes would give you half this speedup or about a 3
percent speeup.  So there is no overwhelmingly strong motivation to
provide fine tuning of these sizing although it could make a
significant difference if you had almost enough memory to double
the hash table but just barely under.    In this case you might be
missing about 5% which you might consider significant (I know I will
work pretty hard to get a 5% speedup in my chess program)

Anyone can do this experiment.  Set your hash table size small and
time about 10 random positions pretty deeply.  Double your hash table
size and time them again.  Note the average speedup of each position
separately and average them together.  At some point you will stop
seeing a benefit (when the tables are big enough.)   When you reach
this point, you could do another test with a much longer search and
you will see that you could indeed benefit from even bigger tables.
The deeper the search, the bigger the tables you will need to get
maximum benefit.  There is always a very sharp point of diminishing
returns once they are large enough and extra memory after this is

To find the approximate best minimal size at 40/2, you should set up
positions that take between 5 and 10 minutes to achieve some depth
and try to figure out what the optimal size is to accomodate this.
Many searches at 40/2 (especially with toot'ing) will take 10 minutes
or more depending on your opponents time allocation style and the
programs time control algorithms.

- Don

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