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Javaforum looking into the memory
- 1. Out Of Memory Analyze Your Java Heap
- 2. GC silver bullet • Unreferenced memory cleared Fragmentation cleanup Incorrect understanding of data / structures • Packratting
- 3. Data / Structures • Primitives vs Objects Byte vs byte • Overhead of structure • LinkedList vs ArrayList JVM settings • UseCompressedOops, UseCompressedStrings
- 4. Packratting • Unintentionally storing data Unbounded caches • Classloaders • Containers / plugins ThreadLocals • Statics
- 5. What now Avoid situation ;-)
- 6. Snapshot • -XX:+HeapDumpOnCtrlBreak -XX:+HeapDumpOnOutOfMemoryError jmap -dump:format=b,file=snapshot.bin pid • jconsole
- 8. MAT Eclipse Memory Analyzer Eclipse Memory Analyzer • Wizards Compare • Explore Query
- 26. Watch out • Can be large Takes time to produce • Can contain sensitive information
- 27. LInks MAT http://www.eclipse.org/mat/ jconsole http://docs.oracle.com/javase/7/docs/technotes/tools/share/jconsole.html jmap http://docs.oracle.com/javase/7/docs/technotes/tools/share/jmap.html jstat http://docs.oracle.com/javase/7/docs/technotes/tools/share/jstat.html • Java API http://www.eclipse.org/forums/index.php/t/243401/
Editor's Notes
- The data we need might be larger than we realize Storing too much intentionally (Caches) Storing too much unintentionally (forgetting to dereference)
- Primitive types vs objects Referencing same object vs a copy Integer.valueOf...
- Generate a snapshot by sending signal, automatically from keyboard or different external programs.
- Sample program which tries to store short strings by doing substring on large strings (which actually uses the same large char array as the large string.
- Different aspects of using MAT
- Open a snapshot
- Store each snapshot in a separate folder since opening it in MAT will create several index files used by MAT and will soon clutter up a folder with several snapshots.
- Opening can take some time (indexes are generated)
- After startup dirrerent reports can automatically be run.
- MAT shows if a few objects are holding on to most of the memory and makes them suspects.
- Generate class histograms from the histogram button.
- You can now see the number of objects, their shallow heap (memory by the object itself) and retained heap (memory by objects referenced, such as lists, maps etc)
- To the left info about the class / instance can be seen
- Show instances of this class with instances pointing to it.
- Filter histogram with regexp
- Show instances with what it points to
- StoreLargeString is small, but references a List which is large, has an array of 1000000 elements, and 1000000 strings, each referencing an array of 26KB
- Select previous searches into compare basket
- Compare and you can see actual values or differences between queries
- With OQL you can select objects from their type and member values
- Handle the snapshots with care, they can contain sensitive information
- Links. The Java API mainly points to discussion of how to use.