GC Time by cause/phase

[Chris Conroy]

(I tried sending this to hotspot-dev 5 days ago but it still has not been
approved by the moderator. Perhaps this is the more appropriate list?)

I'm trying to get detailed GC timing metrics. In particular, I'm interested
in tracking time spent in stop the world GC vs parallel GC in my
application. I was hoping that the GarbageCollectionNotificationInfo would
give me what I need, but I'm having some trouble. I can get total wallclock
time per collector by polling the Garbage Collector MX Beans or watching
the corresponding notifications. However, I'd like to be able to record
time spent in the different phases within the collector (e.g. rescan
stop-the-world vs. concurrent sweep)

I see that recently GC Cause tracing was fixed for CMS in hotspot (
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8008500).

I tried testing jdk7u45 to see if I could get the corresponding cause
information in the GarbageCollectionNotificationInfo MBean notification.
Confusingly, the cause for CMS runs reported via this notification are
always "No GC" even though running the same code with -XX:+PrintGCDetails
shows me the various CMS phases like Initial Mark or Concurrent Mark.

I'm not familiar with the hotspot code base, but it would seem that the
cause should propagate up through gcNotifier.cpp and into this
notification, but there is definitely a disconnect between the verbose GC
logs and the notifications I get.

It doesn't look like jstat or any of the other standard tools give this
information, but it would be really valuable to understand how much time
we've been completely stopped vs. doing parallel GC as the former has
important implications for an application trying to service requests on a
tight deadline.

Is this a bug in what hotspot sends to GarbageCollectionNotificationInfo?
Is there some other programmatic way of getting this information, or do I
just need to tail and parse the JVM GC Logs?
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