I wrote a program that uses a timed thread to collect data from a C producer (using FFI). The number of threads in C producer are fixed (and created at init). One haskell timer thread uses threadDelay to run itself on timed interval. When I look at RTS output after killing the program after couple of timer iterations, I see number of worker tasks increasing with time. For example, below is an output after 20 iterations of timer event: MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) Task 1 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) .......output until task 37 snipped as it is same as task 1....... Task 38 (worker) : 0.07s ( 0.09s) 0.00s ( 0.00s) Task 39 (worker) : 0.07s ( 0.09s) 0.00s ( 0.00s) Task 40 (worker) : 0.18s ( 10.20s) 0.00s ( 0.00s) Task 41 (worker) : 0.18s ( 10.20s) 0.00s ( 0.00s) Task 42 (worker) : 0.18s ( 10.20s) 0.00s ( 0.00s) Task 43 (worker) : 0.18s ( 10.20s) 0.00s ( 0.00s) Task 44 (worker) : 0.52s ( 10.74s) 0.00s ( 0.00s) Task 45 (worker) : 0.52s ( 10.75s) 0.00s ( 0.00s) Task 46 (worker) : 0.52s ( 10.75s) 0.00s ( 0.00s) Task 47 (bound) : 0.00s ( 0.00s) 0.00s ( 0.00s) After two iterations of timer event: MUT time (elapsed) GC time (elapsed) Task 0 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) Task 1 (worker) : 0.00s ( 0.00s) 0.00s ( 0.00s) Task 2 (worker) : 0.07s ( 0.09s) 0.00s ( 0.00s) Task 3 (worker) : 0.07s ( 0.09s) 0.00s ( 0.00s) Task 4 (worker) : 0.16s ( 1.21s) 0.00s ( 0.00s) Task 5 (worker) : 0.16s ( 1.21s) 0.00s ( 0.00s) Task 6 (worker) : 0.16s ( 1.21s) 0.00s ( 0.00s) Task 7 (worker) : 0.16s ( 1.21s) 0.00s ( 0.00s) Task 8 (worker) : 0.48s ( 1.80s) 0.00s ( 0.00s) Task 9 (worker) : 0.48s ( 1.81s) 0.00s ( 0.00s) Task 10 (worker) : 0.48s ( 1.81s) 0.00s ( 0.00s) Task 11 (bound) : 0.00s ( 0.00s) 0.00s ( 0.00s) Haskell code has one forkIO call to kick off C FFI - C FFI creates 8 threads. Runtime options are "-N3 +RTS -s". timer event is kicked off after forkIO. It is for the form (pseudo-code): timerevent <other arguments> time = run where run = do threadDelay time >> do some work >> run where <other variables defined for run function> I also wrote a simpler code using just timer event (fork one timer event, and run another timer event after that), but didn't see any tasks in RTS output. I tried searching GHC page for documentation on RTS output, but didn't find anything that could help me debug above issue. I suspect that timer event is the root cause of increasing number of tasks (with all but last 9 tasks idle - I guess 8 tasks belong to C FFI, and one task to timerevent thread), and hence, memory leak. I will appreciate pointers on how to debug it. The timerevent does forkIO a call to send collected data from C FFI to a db server, but disabling that fork still results in the issue of increasing number of tasks. So, it seems strongly correlated with timer event though I am unable to reproduce it with a simpler version of timer event (which removes mvar sync/callback from C FFI).