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The RTS has a output TASKS which is not documented:

http://downloads.haskell.org/\~ghc/master/users-guide/runtime_control.html\#rts-options-to-produce-runtime-statistics

http://downloads.haskell.org/\~ghc/8.6.3/docs/html/users_guide/runtime_control.html\#rts-options-to-produce-runtime-statistics

Sample of current output:

...

TASKS: 7199 (1 bound, 33 peak workers (7198 total), using -N4)

SPARKS: 0(0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

INIT time 0.004s ( 0.003s elapsed)
MUT time 369355.862s (36159.798s elapsed)
GC time 2539.600s (219.713s elapsed)
EXIT time 0.025s ( 0.028s elapsed)
Total time 371895.491s (36379.541s elapsed)

Alloc rate 7,192,933 bytes per MUT second

Productivity 99.3% of total user, 99.4% of total elapsed