devWorks Benchmarking article

This is a great article from the IBM devWorks site about Java performance benchmarking. I'm just capturing some notes in this entry.

Measuring time:

1. System.currentTimeMillis() - gets the "wall clock" time, but the updates from the OS are hardware dependent and may only occur every ~10 ms. call to OS returns instantly
   
2. System.nanoTime() - returns a differential time, measured in microseconds, bu the call to the OS itself can take microseconds.
3. ThreadMXBean - JMX extension that offers to read a Thread's CPU usage (may be misleading due to I/O and it's uage may be expensive)

Code warmup:

• Class loading can be observed via ClassLoadingMXBean
• Most VMs run the code for while in interpreted mode to gether stats before performing JIT compilation. Sun's Hot Spot defaults: 1500 time for client VMs, 10,000 for server. Could use CompilationMXBean to measure JIT time, but impl is hosed. Alternative is to watch stdout with -XX:+PrintCompilation JVM option