[Haskell-cafe] GHC strange behavior

Stefan O'Rear stefanor at cox.net
Sat Feb 16 19:21:12 EST 2008

On Sun, Feb 17, 2008 at 03:07:15AM +0300, Ruslan Evdokimov wrote:
> 2008/2/17, Jonathan Cast <jonathanccast at fastmail.fm>:
> >
> > Wild guess?  If you leave o as a thunk, to be evaluated once the
> > program has e, then it has numbers, so you keep the entire 10-million
> > entry list in memory.  Evaluating e and o in parallel allows the
> > system to start garbage collecting cons cells from numbers much
> > earlier, which reduces residency (I'd've been unsuprised at more than
> > two orders of magnitude).  Managing the smaller heap (and especially
> > not having to copy numbers on each GC) then makes the garbage
> > collector go much faster, so you get a smaller run time.
> >
> But I also tested it on P-IV 3.0 with HT and 1GB (single core) running
> Windows-XP (ghc 6.8.2), and it works fine (fast & low GC) in all three
> cases without significant difference. Sure it didn't runs faster with
> -N2 'cause it's not dual-core.

This makes perfect sense - -N2 tells GHC to use two threads, and if you
run two threads on a single-processor system it's implemented by running
the threads alternatingly (around 100/s for modern Linux, probably
similar for other systems).  Thus, the two evaluations never get more
than a hundreth of a second out of step, and memory usage is still low.

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