JVM Performance

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Notes from http://www.infoq.com/presentations/JVM-Performance-Tuning-twitter

  • Biggest contributor to latency is the Garbage Collector
  • Areas of Tuning : Memory, Lock Contention, CPU Usage, I/O
  • Areas of Memory Tuning:
    • Memory footprint tuning
    • Allocation rate tuning
    • GC tuning

Memory footprint tuning

  • It's all about ensuring that the GC has less work to do
  • One of the classic symptoms is the OutOfMemoryError
  • Why ? It can be too much data, or the data representation is "fat", or there is a memory leak in the app code
  • What is too much data ?
  • Look at the GC logs. Turn on the verbosegc option
  • Look at the Full GC messages : [Full GC $before->$after($total), $time secs]
  • If more memory can be added, add it.
  • Do we actually need ALL that data in memory, consider using techniques such as LRU cache & Soft References
  • Data representation itself
  • JVM object header is two machine words. i.e. new Object() will consume 16 bytes on a 64 bit JVM
  • Simplest possible array decl : new byte[0] will take 24 bytes : 16 + 4 bytes (length) + 4 (round off to next multiple of 8)
  • e.g. avoid usage of primitive Wrappers.
  • Also not possible to know how the libraries we use are implemented - unless they are profiled.