Hey,

when going through Simon-nofib-notes, I stumbled over this thread from March 2017, when I hadn't yet subscribed to this list: https://mail.haskell.org/pipermail/ghc-devs/2017-March/013887.html.

Joachim and Simon were trying to pin-point seemingly random regressions and improvements of ~5% to `binary-trees`.

It's hard to say in retrospect, but I suspect this is the same effect I experienced in #15333 and I'm about to fix in #15999, e.g. that small changes to allocations lead to big, uncorrelated jumps in runtime performance due to GC.

Just wanted to record this here for posterity.

Cheers
Sebastian

Hi,

Am Dienstag, den 07.03.2017, 22:55 +0000 schrieb Simon Peyton Jones via
ghc-devs:
> > But: binary-trees runtime increases by 5%.
> 
> David: might you look to see if there is any obvious reason for this
> regression?  We could just accept it, but it's always good to know
> why, and to document it.

Turns out that my commit
Add rule mapFB c (λx.x) = c
fixed that regression:
https://perf.haskell.org/ghc/#revision/2fa44217c1d9722227297eefb0d6c6aed7e128ca

Maybe there is just a performance cliff there, and these jumps don’t
really mean anything.

Greetings,
Joachim