In https://gitlab.haskell.org/ghc/ghc/-/issues/15897#note_345881 @jwaldmann shows an `-RTS -s` output reproduced here: (this ticket is about counting gen0 parallel collections, not the absurd GC time and productivity) ``` ... +RTS -qn1 -N8 -M10g -p -h -RTS 26,442,284,496 bytes allocated in the heap 2,140,046,565,680 bytes copied during GC 734,162,344 bytes maximum residency (97 sample(s)) 5,612,520 bytes maximum slop 1483 MiB total memory in use (0 MB lost due to fragmentation) Tot time (elapsed) Avg pause Max pause Gen 0 10019 colls, 10019 par 4024.705s 4.627s 0.0005s 0.0027s Gen 1 97 colls, 96 par 75.138s 48.021s 0.4951s 1.5685s Parallel GC work balance: 12.17% (serial 0%, perfect 100%) TASKS: 24 (1 bound, 23 peak workers (23 total), using -N8) SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled) INIT time 0.008s ( 0.005s elapsed) MUT time 0.000s ( 8.847s elapsed) GC time 4081.967s ( 34.590s elapsed) RP time 0.000s ( 0.000s elapsed) PROF time 17.875s ( 18.058s elapsed) EXIT time 0.022s ( 0.018s elapsed) Total time 79.236s ( 61.518s elapsed) Alloc rate 0 bytes per MUT second Productivity -5051.7% of total user, 43.7% of total elapsed ``` Note that it is run with `-qn`, which should disable parallel collections in gen0, however the report shows all gen0 collections are parallel. This seems to be very similar to https://gitlab.haskell.org/ghc/ghc/-/issues/19147/ In fact, I am almost certain the error is at <https://gitlab.haskell.org/ghc/ghc/-/blob/ghc-9.0.1-release/rts/sm/GC.c#L609>, where this is not a valid way of determinining whether this was a parallel collection.