Re: [RFC] Enhanced Garbage Collection Probe Points

[Lukas Berk <lberk at redhat dot com>]

* Jon VanAlten <jvanalte@redhat.com> [2012-08-08 16:38]:
[...]
> 
> Hi Lukas,
> 
> I've had a look at the patch, and the rest of the thread (especially the
> tapset you posted, which made things a LOT more clear so thanks for that).
> I hope that you'll continue to refine the tapset and contribute that as
> well so that us java hackers who are brave enough to play with Systemtap
> have the nice friendly probe names and variables to work with!
Thanks for taking a look, I'll make sure to include it in future
revisions, ideally I'll have a gc_details.stp.in file that will have the
.so path change based on the system it's being compiled for. (similar to
the hotspot{,_jni}.stp.in and jstack.stp.in files already included)
> 
> The patch itself seems fine, although I'll just have to trust you that the
> probes are in the right place as I'm not really a hotspot hacker myself.
> Really I would hope that someone who is more familiar with the GC code
> base will comment from that perspective.
> 
> I assume that none of these probes require any special VM args to function?
> A related question, and this might be hard to answer being rather open
> ended, but are you aware of any VM args that would affect/interfere with
> the functioning of these probes?
The only arguments that I can think of that will immediately effect
these probes are:
-XX:+UseSerialGC
-XX:+UseParallelGC
-XX:+UseParallelOldGC
-XX:+UseParNewGC
-XX:+UseConcMarkSweepGc
-XX:+UseG1GC
which will force the JVM to use the designated type of garbage
collection.  There may be more I'm missing (please let me know if there
are).
> 
> There is a begin and and end probe for some types of collection and even
> phases within collection, so it seems odd to me that for G1 stop-the-world
> collection there is not.  Am I missing something?

My thought in adding the begin/end probes was to be able to track
parallel operations and tie a specific pid to an collection/scavenge to
an object.  That wouldn't be needed in serial operation.  However, now
that I think about it, it could be useful to mark the beginning and end
of 'serial' collections from a timing perspective, and its trivial to
add (I'll include it in a revised patch).
> 
> Other than that; great start, this definitely has stuff interesting from a
> Thermostat perspective and the potential for more :D
> 

Thanks for the comments,

Lukas

[1] - http://dacapobench.org/

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