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

Author: Dieter Buerssner

Date: 11:41:45 07/16/03

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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.

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.

Regards,
Dieter




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