All, A recent SO question [1] led me to do a quick test on hard-coding RTS options as suggested in the manual rts-hooks section [2]. Unfortunately the timing tests indicate "ghc_rts_opts" isn't being used; tests with flags besides -N (ex: "-H1024m") also show the ghc_rts_opts being ignored. Can someone shed light on what's happening? A grep of ghc source shows no users of ghc_rts_opts, though that isn't definitive. Cheers, Thomas -------- $ cat rts-opts.h char *ghc_rts_opts = "-N"; $ ghc --make -O2 -threaded hashPar.hs rts-opts.h -fforce-recomp [1 of 1] Compiling Main ( hashPar.hs, hashPar.o ) Linking hashPar ... $ time ./hashPar 10MB 10MB 10MB 10MB ["10MB","10MB","10MB"] real 0m2.411s $ time ./hashPar 10MB 10MB 10MB 10MB +RTS -N ["10MB","10MB","10MB"] real 0m1.690s ---------- [1] http://stackoverflow.com/questions/3704430/why-do-you-have-to-use-both-a-com... [2] http://haskell.org/ghc/docs/6.12.2/html/users_guide/runtime-control.html#rts...