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

Author: Gerd Isenberg

Date: 12:40:07 07/16/03

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On July 16, 2003 at 14:41:45, Dieter Buerssner wrote:

>Just few comments about the thread.
>An interesting test would be, to do lmbench type linked list test with Vincent's
>idea of real random access. I may try it out later. No PRNG calls will be
>needed. The linked list will be initialized "pseudo randomly". In this case, it
>would mean, that it will not be too close to real random, because in one cycle
>every memory adress will be read once. (This could easily happen anyway, with
>not so decent PRNGs).
>An perhaps interesting comment from lmbench source:
>       /*
>        * First create a list of pointers.
>        *
>        * This used to go forwards, we want to go backwards to try and defeat
>        * HP's fetch ahead.
>        *
>        * We really need to do a random pattern once we are doing one hit per
>        * page.
>        */
>So, the authors did not seem too confident with the sequential like access? Or
>did I misunderstand.

Dieter, sorry for ignoring this in my post below.
Yes i see, but to do exactly Vincent's sequence, calling RanrotA 100 million
times in a loop, you need a rather huge list ;-)


>The PRNG Vincent uses is fine. I will do some tests on it. Lagged Fibonacci type
>generators don't have problems with mod (often rand() uses a linear congruential
>generator, which can have severe problem, especially when used with mod. Anyway,
>for this sort of test, I think even very bad PRNGs would do well. There is no
>way, the hardware can guess the access pattern.

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