Is there a way to get "Dominator Tree"-like functionality from a running JVM?

Question

Recently, I've been digging in to JVM heap dumps using Eclipse MAT. I like it, but the one feature that I seem to use the most is the Dominator Tree. Eclipse's example screenshot:

Anyways, I find that a lot of the time, I usually get the most value out of just looking at that table and getting the first few entries. Since the turnaround time for getting this is:

1. Create Heap Dump (jcmd <pid> GC.heap_dump)
2. Download/Pull heapdump to a location (MAT isn't installed on our servers)
3. Run Eclipse MAT's ParseHeapDump.sh tool to build the various trees
4. Open MAT, click Dominator Tree icon.
5. Analyze

Is there a way to get this equivalent information off of a running JVM programmatically? I'd like to run some kind of gather_dominators.sh <pid> script on a host and get the Top X Objects from a JVM, but I don't know where to start.

Answer 1

If by "running jvm" You meant - "getting the info without doing stop-world heap-dump" then the obvious answer is: in order to do such thing without "full-scan" - the data needs to be collected throughout system life-time by tapping creation/release of each object and by maintaining the statistics. You could achieve such things by instrumentation or by using a ready-made custom agents (jol/jamm/etc). Note that many GCs are already doing similar work to collect (and print) statistics. IIRC - newer JVMs even keep track of such info within the class-metadata area (so getting statistics is instant).

• https://github.com/google/allocation-instrumenter (google-allocation-instumenter)
• http://blog.javabenchmark.org/2013/07/compute-java-object-memory-footprint-at.html (with JAMM)
• https://github.com/jbellis/jamm (JAMM src)
• In Java, what is the best way to determine the size of an object? (JOL/etc here)
• http://www.javaworld.com/article/2074458/core-java/estimating-java-object-sizes-with-instrumentation.html (short DYI guide)
• https://www.youtube.com/results?search_query=Understanding+Java+GC (webCast on how GC traverses objects for similar purposes)

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On other hand - if You're fine to grab a heap-dump (which should be fine on any production system with any proper node-redundancy in place, designed for handling unavoidable Sun-JVM stop-world GC pauses), then Jhat, MAT-api, YourKit and Jol are probably Your best friends:

• Programmatically analyze java heap dump file
• How to analyse the heap dump using jmap in java

It is important to note that currently-existing heap-dump format loses the info about actual sizes of objects, so all tools (MAT/etc) are just trying to GUESS it properly:

• http://shipilev.net/blog/2014/heapdump-is-a-lie/ (What Heap Dumps Are Lying To You About, by Aleksey Shipilёv)

HTH :)