GHC 8.2 GC time stats going negative

[Ben Gamari]

Harendra Kumar <harendra.kumar at gmail.com> writes:

> Hi,
>
> I am trying to use the mutator_cpu_ns and mutator_elapsed_ns from GHC 8.2
> RTSStats structure retrieved using getRTSStats API documented here -
> https://hackage.haskell.org/package/base-4.10.0.0/docs/GHC-Stats.html .
>
> I am seeing that the values of these stats retrieved about 30 ms later are
> lower than the values retrieved earlier. Sometimes even after 100 ms the
> values decrease from the previous ones. However, as the time duration
> between two calls increase this becomes less and less likely.
>
> Is this an expected behavior or am I using it incorrectly? If it is
> expected, why does it happen, what is the underlying mechanism that makes
> it happen?
>
Hmm, interesting. I don't believe this should happen and I do know of
bugs in the RTS's time accounting at exit-time (manifesting in profiling
issues, e.g. #14257) however what you describe doesn't seem to fit that
description. Do you have a simple repro? Is there anything special about
your program (e.g. lots of FFI calls)?

Cheers,

- Ben
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