It seems that the problem you have is that moving to the multithreaded runtime imposes an overhead on the communication between your two threads, when run on a *single CPU*. But performance on a single CPU is not what you're interested in - you said you wanted parallelism, and for that you need multiple CPUs, and hence multiple OS threads.
Well, I'm interested in getting an absolute speedup. If the threaded performance on a single core is slightly slower than the non-threaded performance on a single core, that would be OK provided that the threaded performance using multiple cores was better than the same non-threaded baseline. However, it doesn't seem to work like that at all. In fact, threaded on multiple cores was _even_slower_ than threaded on a single core! Here are some figures: ghc-6.8.2 -O2 apply MVar strict thr-N2 thr-N1 silicium 7.30 7.95 7.23 15.25 14.71 neghip 4.25 4.43 4.18 6.67 6.48 hydrogen 11.75 10.82 10.99 13.45 12.96 lobster 55.8 51.5 57.6 76.6 74.5 The first three columns are variations of the program using slightly different communications mechanisms, including threads/MVars with the non-threaded RTS. The final two columns are for the MVar mechanism with threaded RTS and either 1 or 2 cores. -N2 is slowest.
I suspect the underlying problem in your program is that the communication is synchronous. To get good parallelism you'll need to use asynchronous communication, otherwise even on multiple CPUs you'll see little parallelism.
I tried using Chans instead of MVars, to provide for different speeds of reader/writer, but the timings were even worse. (Add another 15-100%.) When I have time to look at this again (probably in the New Year), I will try some other strategies for communication that vary in their synchronous/asynchronous chunk size, to see if I can pin things down more closely. Regards, Malcolm