[erlang-questions] benchmarks game harsh criticism

Isaac Gouy <>
Fri Nov 30 21:17:53 CET 2007


--- Bengt Kleberg <> wrote:
-snip-
> quotes from "Timing Trials, or, the Trials of Timing" as per request:
> "* Memory-related issues and the effects of memory hierarchies are 
> pervasive: how memory is managed, from hardware caches to garbage 
> collection, can change runtimes dramatically. Yet users have no
> direct 
> control over most aspects of memory management. "
> 
> "We started to construct a table with three dimensions: task, 
> programming language, and machine. Eventually we added the size of
> the 
> problem solved by the program as a fourth dimension, and we changed
> the 
> presentation from tables to graphs. Varying the problem size helped
> us 
> to detect unusual runtime effects, while a graphical presentation 
> highlights patterns and trends in runtime instead of individual 
> performance scores."
> 
> "We designed tests whose runtime should grow linearly with the size
> of 
> the problem: runtime = mÂ?size + b. Thus, if we choose size to be
> large 
> enough to justify ignoring the fixed overhead (b), the log-log plot 
> should show a straight line of unit slope. Exceptions indicate
> anomalous 
> behavior that deserves further attention."


(Incidentally what they meant by anomalous behaviour is something like
this: "the line connecting C runtimes appears absolutely horizontal.
... This happens because the optimizer eliminates the entire loop,
replacing it by sum = n.")


Let me suggest to you that the paragraphs you quote are descriptions
not recommendations. 

Here's a recommendation: "... we advise all who want to know which
version of a program will run faster to construct test programs and
find out the truth for their language processor and machine."

Here's another: "It does seem wise to take all such experiments
­including these ­with a large grain of salt."


> Now it is your turn. could you quote the exact words where they say
> that 4 inputs and a spread of x10 is good enough?

There is no such statement, nor have I claimed that there is: I have
described the spread shown in the tests - "we can see that they varied
the problem size by < ~10x" 



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