Better perf

Sebastian Graf sgraf1337 at
Fri Dec 14 15:07:50 UTC 2018


when going through Simon-nofib-notes, I stumbled over this thread from
March 2017, when I hadn't yet subscribed to this list:
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.


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