JVM Memory Management (2)

This is a study note from “Everything I Ever Learned about JVM Performance Tuning @Twitter”.

• Adaptive Sizing Policy:
  Throughput collectors an automatically tune themselves:
• -XX:+UseAdaptiveSizePolicy
• -XX:+MaxGCPauseMillis=…(i.e. 100)
• -XX:+GCTimeRatio=…(i.e. 19)
• tune young generation tool:
• enable -XX:+PrintGCDetails, -XX:+PrintHeapAtGC, and -XX:+PrintTenuringDistribution
• watch survivor size
• watch the tenuring threshold; it might need to tune it to tenure long lived objects faster.
• some rules:
• Too many live objects during young generation GC:
  Reduce NewSize, reduce survivor spaces, reduce tenuring threshold.
• Too many threads:
  Find the minimal concurrency level, or split the service into serval JVMs.
• Eden should be big enough to hold more than one times transactions. In this case, there is no stop-the-world and through output would be big.
• Each survivor should be big enough to maintain alive objects and aged objects.
• Increasing threshold will put long aged objects to old generation asap to release more space to survivor.
• Now we use 64-bit JVM, 64-bit pointer will cause CPU buffer is smaller than 32-bit pointer. Involving -XX:+UseCompressedOops will compress 64-bit pointer to 32-bit pointer, but it still will use 64-bit memory space.
  Object stored in memory split into 3 parts:
• header: mark word + klass pointer
• mark word stores running data for object itself.
• klass pointer points to the object’s class metadata.
• instance data:

• padding: 0 <= padding <= 8
  (header + instance data + padding) % 8 == 0