web ops

Java GC Basics

Basic notes on Java garbage collection that every system engineer dealing with Java in production should know.

• Always run with gc logging enabled.
• Watch for promotion failures in the gc log.  If you are have any, the heap is fragmented, restart the process.
• GC time seems to be proportional to size of the heap.  GC’s will be slow with a 10GB heap size.
• The CMS collector has better concurrency, but it is VERY slow.  To use it your application may have to do a lot of manual object pooling.
• 10% or 20% of time spent in GC is reasonable.  50% is not.  99% means you are out of memory.
• Be sure to use -server so that multiple cores are used for Full GC’s.
• -Xmx is the really important parameter.  -Xms not so much.