Hi, Am Sonntag, den 06.05.2018, 16:41 +0200 schrieb Andreas Klebinger:
With a high number of NoFibRuns (30+) , disabling frequency scaling, stopping background tasks and walking away from the computer till it was done I got noise down to differences of about +/-0.2% for subsequent runs.
This doesn't eliminate alignment bias and the like but at least it gives fairly reproducible results.
That’s true, but it leaves alignment bias. This bit my in my work on Call Arity, as I write in my thesis: Initially, I attempted to use the actual run time measurements, but it turned out to be a mostly pointless endeavour. For example the knights benchmark would become 9% slower when enabling Call Arity (i.e. when comparing (A) to (B)), a completely unexpected result, given that the changes to the GHC Core code were reasonable. Further investigation using performance data obtained from the CPU indicated that with the changed code, the CPU’s instruction decoder was idling for more cycles, hinting at cache effects and/or bad program layout. Indeed: When I compiled the code with the compiler flag -g, which includes debugging information in the resulting binary, but should otherwise not affect the relative performance characteristics much, the unexpected difference vanished. I conclude that non-local changes to the Haskell or Core code will change the layout of the generated program code in unpredictable ways and render such run time measurements mostly meaningless. This conclusion has been drawn before [MDHS09], and recently, tools to mitigate this effect, e.g. by randomising the code layout [CB13], were created. Unfortunately, these currently target specific C compilers, so I could not use them here. In the following measurements, I avoid this problem by not measuring program execution time, but simply by counting the number of instructions performed. This way, the variability in execution time due to code layout does not affect the results. To obtain the instruction counts I employ valgrind [NS07], which runs the benchmarks on a virtual CPU and thus produces more reliable and reproducible measurements. Unpleasant experience. Cheers, Joachim -- Joachim Breitner mail@joachim-breitner.de http://www.joachim-breitner.de/