Reported GC time is way too high

[Eax Melanhovich]

Hello.

I'm using Kamon with Datadog. Here is how one of the charts looks like:

http://imagizer.imageshack.com/img909/8697/zuXoSL.png

Which units are these? I doubt that this is milliseconds, since if I had
such GC time, I would notice :) It's also doubtful that we are talking
about microseconds.

I took a look at

./kamon-system-metrics/src/main/scala/kamon/metrics/GCMetrics.scala

Although the code is quite complicated it looks like unit should be
milliseconds.

Could it be a bug? Or maybe I'm doing something wrong?

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Best regards,
Eax Melanhovich
http://eax.me/

[Ivan Topolnjak]

Hello Eax, welcome to our community!

The time you are seeing is indeed in milliseconds and I guess that the reason for being so high is that it is the accumulated [1] time since the startup of your JVM. Probably if you zoom out to hours or even days depending on how long it has been running, you should see "normal" collection times of few milliseconds and ramping up over the lifetime of the JVM running the app.

Now that I think about it, it would be nice to report the difference in GC time since the last tick, instead of the accumulated value that it is reporting now. It should be a lot easier to identify longs stalls related to GC by doing it that way as well as identifying when the GCs happen. What do you think about that change?

[1] https://docs.oracle.com/javase/7/docs/api/java/lang/management/GarbageCollectorMXBean.html#getCollectionTime()

[Eax Melanhovich]

Sounds reasonable. In fact, accumulated value was last thing I
expected :)

[Diego Parra]

Hi Eax,  

just in case that you want follow this problem, we open a new issue [1]

[1] https://github.com/kamon-io/Kamon/issues/135

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[Eax Melanhovich]

Thank you!