what could be the reason for my program (compiled with -threaded) crashing with +RTS -N2 it seems to work fine with "-N1". I compiled with ghc-6.8.1 (installed from binary package). I am using forkIO, some MVars, some IORefs, some Channels. It seems -prof -threaded does not work: ghc-6.8.1: combination not supported: Threaded/Profiling When I run the executable under gdb I get Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 0xb6afeb90 (LWP 7801)] 0x08138d8c in checkBlackHoles () This behaviour occurs both on a dual core PC and on an single core hyperthreaded PC, both running openSuse-10.3 I cannot tell whether this is an regression from 6.6 (I cannot easily backport my code). Any hints appreciated, Johannes Waldmann.