I posted this issue on StackOverflow today. A brief recap: In the case when C FFI calls back a Haskell function, I have observed sharp increase in total time when multi-threading is enabled in C code (even when total number of function calls to Haskell remain same). In my test, I called a Haskell function 5M times using two scenarios (GHC 7.0.4, RHEL5, 12-core box): - Single-threaded C function: call back Haskell function 5M times - Total time 1.32s - 5 threads in C function: each thread calls back the Haskell function 1M times - so, total is still 5M - Total time 7.79s - Verified that pthread didn't contribute much to the overhead by having the same code call a C function instead, and compared with single-threaded version. So, almost all of the increase in overhead seems to come from GHC runtime. What I want to ask is if this is a known issue for GHC runtime? If not, I will file a bug report for GHC team with code to reproduce it. I don't want to file a duplicate bug report if this is already known issue. I searched through GHC trac using some keywords but didn't see any bugs related to it. StackOverflow post link (has code and details on how to reproduce the issue): http://stackoverflow.com/questions/8902568/runtime-performance-degradation-f...