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Subject: Re: Source code to measure it - there is something wrong

Author: Robert Hyatt

Date: 15:25:51 07/16/03

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On July 16, 2003 at 16:12:45, Dieter Buerssner wrote:

>On July 16, 2003 at 15:40:07, Gerd Isenberg wrote:
>
>>Yes i see, but to do exactly Vincent's sequence, calling RanrotA 100 million
>>times in a loop, you need a rather huge list ;-)
>
>Hi Gerd, for 100 millions, I would need 400 Mb RAM (which I can). I need
>100e6*sizeof(pointer). But thatis not really the point. My idea (one alternative
>at least) is to start with a cyclic linked list, that just connects the elements
>in order at first, and last entry goes back to start. Then use a typical card
>shuffling routine, to randomize this list. If the mem is over 400 Mb, it would
>mean, that we don't visit each cell. If it is lower, we will visit the cells
>serveral timed. I would suspect, as long as memory is much bigger than cache
>sizes, it makes little difference. But does it make a difference to lmbench type
>sequential testing (with the same linked list method)?
>
>And, after all, we use virtual memory nowadays. Doesn't this include one more
>indirection (done by hardware). Without knowing much about it, I wouldn't be
>surprized, that hardware time for those indirections is needed more often with
>the random access style pattern.

You are talking about the TLB.

The memory mapping hardware needs two memory references to compute a real
address before it can be accessed.  The TLB keeps the most recent N of these
things around.  If you go wild with random accessing, you will _certainly_
make memory latency 3x what it should be, because the TLB entries are 100%
useless.  Of course that is not sensible because 90+ percent of the memory
references in a chess program are _not_ scattered all over memory.

>
>BTW. To shuffle a deck of N cards, one needs N calls to the PRNG. Often people,
>who try this first, do it rather wrong. They randomly choose 2 indices, and swap
>those cards. This is a very inefficient method, and even with many more than N
>calls to the PRNG, the deck is not shuffled well.
>
>I did not start yet. When I have the results, I can think about possible
>problems with mod, loop unrolling, etc. (I will use loop unrolling for the
>random linked list test).
>
>Cheers,
>Dieter



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