Reducing JVM pause time > 1 second using UseConcMarkSweepGC

Question

I\'m running a memory intensive app on a machine with 16Gb of RAM, and an 8-core processor, and Java 1.6 all running on CentOS release 5.2 (Final). Exact JVM details are:

Answer 1

Since you mention your desire to cache, I'm guessing that most of your huge heap is occupied by that cache. You might want to limit the size of the cache so that you are sure it never attempts to grow large enough to fill the tenured generation. Don't rely on SoftReference alone to limit the size. As the old generation fills with soft references, older references will be cleared and become garbage. New references (perhaps to the same information) will be created, but cleared quickly because free space is in short supply. Eventually, the tenured space is full of garbage and needs to be cleaned.

Consider adjusting the -XX:NewRatio setting too. The default is 1:2, meaning that one-third of the heap is allocated to the new generation. For a large heap, this is almost always too much. You might want to try something like 9, which would keep 9 Gb of your 10 Gb heap for the old generation.

Answer 2

I haven't personally used such a huge heap but I've experienced very low latency in general using following switches for Oracle/Sun Java 1.6.x:

-Xincgc -XX:+UseConcMarkSweepGC -XX:CMSIncrementalSafetyFactor=50
-XX:+UseParNewGC
-XX:+CMSConcurrentMTEnabled -XX:ConcGCThreads=2 -XX:ParallelGCThreads=2
-XX:CMSIncrementalDutyCycleMin=0 -XX:CMSIncrementalDutyCycle=5
-XX:GCTimeRatio=90 -XX:MaxGCPauseMillis=20 -XX:GCPauseIntervalMillis=1000

The important parts are, in my opinion, the use of CMS for tenured generation and ParNewGC for young generation. In addition, this adds a pretty big safety factor for CMS (default is 10% instead of 50%) and request short pause times. As you're targeting for 25 ms response time, I'd try setting -XX:MaxGCPauseMillis to even smaller value. You could even try to use more than two cores for concurrent GC but I would guess that is not worth the CPU usage.

You should probably also check the HotSpot JVM GC cheat sheet.