Hello, I'm currently developing some applications with explicit threading using forkIO and have strange behaviour on my freshly installed Ubuntu Karmic 9.10 (Kernel 2.6.31-14 SMP). Setup: Machine A: Quadcore, Ubuntu 9.04, Kernel 2.6.28-13 SMP Machine B: AMD Opteron 875, 8 cores, 2.6.18-164 SMP- (some redhat) Machine C: Dual-Core, Ubuntu 9.10, Kernel 2.6.31-14 SMP Compiler on all machines: ghc 6.10.4 (downloaded from GHCs official website) Program, Compilation, Execution A simple taskqueue with independent tasks and explicit parallelization (hence should deliver more or less perfect speedup). For one core wall-times around 16 are ok, for 2 a bit more than 8 seconds. Since I used the same sources and Makefiles on all machines all files were compiled with -threaded and started with +RTS -N2 -RTS. Testing: Machine A: Ok (meaning works and delivers the expected speedup) Machine B: Ok Machine C: Not ok (with -N2 wall times around 14-15 seconds) Looking at the core usage, for example with htop, I see that the second core is not really used on C. Executing OpenMP programs shows the expected speedup and usage of both cores, hence I do not think its a kind of general linux configuration problem. So, after all the testing I think its either the Linux Kernel or some other component of Ubuntu 9.10. But: Ubuntu is often used and I did not found any information regarding this problem. The simple solution of installing the old version of Ubuntu would probably help but should not be the way to go, should it? I'd be glad for any hints or comments, Michael -- Dipl.-Inf. Michael C. Lesniak University of Kassel Programming Languages / Methodologies Research Group Department of Computer Science and Electrical Engineering Wilhelmshöher Allee 73 34121 Kassel Phone: +49-(0)561-804-6269