I have an application that is largely latency-bound, it operates on hundreds of concurrent DNS lookup tasks, to process workloads of millions to tens of millions such tasks. Each task is spawned via Control.Concurrent.Async.async, and the code is compiled with "-threaded", and runs with the "capability" count set to 4: $ ps -o command= -p $(pgrep -x danescan) danescan ... +RTS -N4 ... The main task collects batches of outputs from the worker tasks and stores the results in a database. With the worker count set to 350, looking at per-thread "ps" output on FreeBSD, I see 24 threads of which 3 look idle, 20 seem to be doing the DNS work (with a few noticeably less than the rest), and one is the main thread doing the database updates: $ ps -o pid,lwp,time,mwchan,msgrcv,msgsnd,state -H -p $(pgrep -x danescan) | sort -k5n PID LWP TIME MWCHAN MSGRCV MSGSND STAT 38083 100763 0:00.00 uwait 0 0 I+ 38083 101152 0:00.00 uwait 0 0 I+ 38083 101922 0:00.00 uwait 0 0 I+ 38083 101343 0:02.29 uwait 27194 27627 I+ 38083 101335 0:04.30 uwait 51689 52517 I+ 38083 101391 0:13.33 uwait 173618 177160 S+ 38083 101917 0:22.63 kqread 295420 300894 S+ 38083 101362 0:28.73 uwait 371419 378350 I+ 38083 101349 0:46.11 uwait 594307 604816 I+ 38083 101352 0:49.65 uwait 641703 653557 S+ 38083 101370 0:53.93 select 699057 712217 S+ 38083 101358 0:58.15 uwait 757636 771437 S+ 38083 101376 1:00.84 kqread 794335 809328 S+ 38083 101375 1:02.31 uwait 806276 821284 S+ 38083 101342 1:02.34 uwait 809367 823963 S+ 38083 101244 1:03.26 select 812395 827329 S+ 38083 101360 1:03.47 uwait 815955 831710 S+ 38083 101356 1:03.03 uwait 818215 833649 I+ 38083 101364 1:03.54 uwait 825807 841300 S+ 38083 101347 1:03.89 uwait 832981 848893 S+ 38083 101334 1:04.42 kqread 836132 851386 S+ 38083 101357 1:06.04 uwait 859308 875754 S+ 38083 101134 1:06.67 kqread 859410 875176 S+ 38083 100655 5:36.98 uwait 11780649 11780685 S+ The number of open UDP DNS query sockets matches what I expect (each active query uses a fresh socket, closing it when the results arrive): $ lsof -n -P -i -a -p $(pgrep -x danescan) | egrep -c '(TCP|UDP).*:53$' 349 How is the actual thread count related to the RTS "-N" option? I did not expect to see 20 threads... -- Viktor.