Performance questions

Java Urban Performance Legends

When is it important to set the default collection sizes?

  • What is the time cost of growing an ArrayList?

  • What is the time cost of growing/rehashing a HashMap with defaults from zero to 'n' entries?
    • initial capacity 16, load factor .75
    • growth algorithm if (size++ >= threshold) resize(2 * table.length)
    • size should always be power of 2
    • 0-1 million objects ? sec
    • 0-10 million objects ? sec
    • 0-20 million objects - it takes three times the current size to rehash since it doubles the internal array and copies the old array - with a 700M heap couldn't rehash 20 M objects without running out of memory
    • 0-20 million objects with 1 GB heap ? sec

Is synchronization expensive?

  • What is the time cost of un-contented synchronization?
  • What is the time cost of contended synchronization?

When would pooling of objects yield significant performance gains?

  • How much memory do empty objects require?
  • How much time do objects take to create?

How close is the size of serialized objects, collections, and primitives to the actual in-memory size?

  • Empty object
  • byte
  • short
  • int
  • long
  • float
  • double
  • boolean
  • char
  • String
  • HashMap
Test environment:
  • Dual core, 2 GHz, 2 GB RAM
  • Java 1.6.0-b105 client mode
  • VM args -Xms700M -Xmx700M
  • Tests warmed up VM
  • Pre-created objects
  • Measured and subtracted iteration time

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