On Sun, Feb 17, 2008 at 03:07:15AM +0300, Ruslan Evdokimov wrote: > 2008/2/17, Jonathan Cast <[EMAIL PROTECTED]>: > > > > 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. Stefan
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