Saturday, July 9, 2011

tips how to size a jvm heap to incur 1 GC/day

(See other post on 1 GC/day)
UBS eq system in Stamford claims to incur a single GC a day. They probably need enough RAM for the heap. 32bit  gives you about 3GB memory, probably insufficient. (Note In order to run 64-bit Java you must have a processor and operating system for 64-bit applications.) 

Just How much RAM is sufficient? Here's an answer from an Oracle/BEA tuning consultant + my own input. (In trading, 16G is a common figure, all dedicated to a single JVM. In a risk engine, each machine has 512GB but runs multiple JVMs.)

1) decide if caching is suitable.
1b) if yes, then make sure caches have size-control and leak-control

2) eliminate obvious memory hogs, reuse objects and pools if possible
3) Profile your engine in load test.
4) tentatively release to production. If it actually uses much higher memory, then investigate why the estimate failed. Possible reasons

- leaks
- cache

Many memory profilers can count how many instances of a particular class (like String.java) there are. If your own class has too many instances, you can add instrumentation to ctor

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