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Subject: Re: Another memory latency test

Author: Dieter Buerssner

Date: 13:51:28 07/17/03

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On July 17, 2003 at 16:11:33, Gerd Isenberg wrote:

>your algorithm confirms roughly Vincent's results!

Yes, I agree. Actually my code was inspired by the issues you raised about
Vincent's test (and the strange things we discussed around omnid_abs with random
loop testing).

>But there is still the question whether this measured times is memory latency
>per definition - i guess not.

Perhaps. I could not find a good definition in lmbench source fast, and neither
in the papers about it (but only read over it very fast).

>In worst cases there is more than memory latency (additional TLB-latency and
>some RAM hardware interface latencies) to get data into a register - maybe a
>question of definition.

Even, when this is the case, those "real" latencies sound pretty uninteresting
from programmer's (and this may imply user's) point of view. They may give
numbers, that are not particually useful for "our" environment. After all, we
are not a group of hardware designers or OS writers.


>Another interesting point is to measure not only the average but the maximum and
>minimum access times (processors performance counter?). Are the accesses  about
>equal, or are there heavy spikes due to some chaotic TLB behaviour?

Yes. Perhaps I try it (or even you might want to try it:-). Performance counters
on x86 are not without pitfalls. Read for example
http://cedar.intel.com/software/idap/media/pdf/rdtscpm1.pdf

I would guess, that the access times are in the big majority about equal. Of
course the random access should yield spikes (towards zero - so actually
"anti-spikes") now and then, because now and then a small offset to the next
memory access will (and should) happen, and the data can be fetched out of
cache.

Many years ago, I knew go32 (one DOS extender, available in source code) quite
well. I fear my memory faded away. But one might have been able to setup it with
no virtual memory, page-tables, etc. Perhaps that was the case for another
alternative to go32 (pmode?). Perhaps, I am totally wrong ...

Cheers,
Dieter



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