GHC 8.2 GC time stats going negative

[Harendra Kumar]

Ben,

I could reproduce it even after removing all FFI calls. I created a ghc
trac ticket, you can find it here:
https://ghc.haskell.org/trac/ghc/ticket/14445. Let me know if any more
information is needed or if explanation about the code/repo is required.

Thanks,
Harendra


On 10 November 2017 at 00:12, Harendra Kumar <harendra.kumar at gmail.com>
wrote:

> Hi Ben,
>
> Yes, there are FFI calls between the two getRTSStats calls. FFI calls are
> C APIs to get some OS stats like clock time or cpu time etc. Now I know
> that this is not expected, let me try to minimize the example and see where
> it goes. I will try removing the FFI calls as well.
>
> -harendra
>
> On 9 November 2017 at 23:49, Ben Gamari <ben at well-typed.com> wrote:
>
>> 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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