RTS changes affect runtime when they shouldn’t

Joachim Breitner mail at joachim-breitner.de
Sat Sep 23 19:06:32 UTC 2017


Am Samstag, den 23.09.2017, 20:45 +0200 schrieb Sven Panne:
> 2017-09-21 0:34 GMT+02:00 Sebastian Graf <sgraf1337 at gmail.com>:
> > [...] The only real drawback I see is that instruction count might
> > skew results, because AFAIK it doesn't properly take the
> > architecture (pipeline, latencies, etc.) into account. It might be
> > just OK for the average program, though.
> > 
> It really depends on what you're trying to measure: The raw
> instruction count is basically useless if you want to have a number
> which has any connection to the real time taken by the program. The
> average number of cycles per CPU instruction varies by 2 orders of
> magnitude on modern architectures, see e.g. the Skylake section in ht
> tp://www.agner.org/optimize/instruction_tables.pdf (IMHO a must-read
> for anyone doing serious optimizations/measurements on the assembly
> level). And these numbers don't even include the effects of the
> caches, pipeline stalls, branch prediction, execution units/ports,
> etc. etc. which can easily add another 1 or 2 orders of magnitude.
> So what can one do? It basically boils down to a choice:
>    * Use a stable number like the instruction count (the
> "Instructions Read" (Ir) events), which has no real connection to the
> speed of a program.
>    * Use a relatively volatile number like real time and/or cycles
> used, which is what your users will care about. If you put a non-
> trivial amount of work into your compiler, you can make these numbers
> a bit more stable (e.g. by making the code layout/alignment more
> stable), but you will still get quite different numbers if you switch
> to another CPU generation/manufacturer.
> A bit tragic, but that's life in 2017... :-}

what I want to do is to reliably catch regressions. What are the odds
that a change to the Haskell compiler (in particular to Core2Core
transformations) will cause a significant increase in runtime without a
 significant increase in instruction count?
(Honest question, not rhetoric).


Joachim Breitner
  mail at joachim-breitner.de
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