NewRatio parameter not working with CMS garbage co

问题:

I switched to CMS collector for my application and throughput of application decreased by half. From GC logs, I see a high frequency of minor GCs happening (aroung 10 per second ). I have allocated a heap size of 4G. The JVM be default is using very small size for young gen (less than 40MB ). I want to try out CMS via increasing the size of young gen. Can you point me to right JVM parameter for this.

• I tried -XX:NewRatio but JVM ignored this parameter and there was no change in young gen sizes
• My java version is java version "1.6.0_14"

回答1:

How did you set -XX:NewRatio, and on which JVM version?

Anyway. It could be ignored if you are also setting -XX:MaxNewSize=size, which is a feature, or if you are also setting -XX:+UseConcMarkSweepGC which is a known bug.

回答2:

It works for me:

The defaults (Java 7 64bit on Windows):

java -Xmx4g -Xms4g -XX:+PrintGCDetails -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -version
java version "1.7.0_07"
Java(TM) SE Runtime Environment (build 1.7.0_07-b10)
Java HotSpot(TM) 64-Bit Server VM (build 23.3-b01, mixed mode)
Heap
 par new generation   total 613440K, used 21813K
  eden space 545344K,   4% used
  from space 68096K,   0% used
  to   space 68096K,   0% used
 concurrent mark-sweep generation total 3512768K, used 0K
 concurrent-mark-sweep perm gen total 21248K, used 2084K

Now specifying the ratio parameters (roughly same result):

java -Xmx4g -Xms4g -XX:+PrintGCDetails -XX:NewRatio=6 -XX:SurvivorRatio=8 -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -version
 par new generation   total 539264K, used 19174K
  eden space 479360K,   4% used
  from space 59904K,   0% used
  to   space 59904K,   0% used
 concurrent mark-sweep generation total 3595136K, used 0K
 concurrent-mark-sweep perm gen total 21248K, used 2084K

And then a totally different new/old and eden/survivor ratio:

java -Xmx4g -Xms4g -XX:+PrintGCDetails -XX:NewRatio=1 -XX:SurvivorRatio=1 -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -version
 par new generation   total 1398144K, used 27965K
  eden space 699136K,   4% used
  from space 699008K,   0% used
  to   space 699008K,   0% used
 concurrent mark-sweep generation total 2097152K, used 0K
 concurrent-mark-sweep perm gen total 21248K, used 2084K

Same works for Java 6 or client VM (just with different defaults).