Standard allocation ratio for Java heap

问题:

I've tried to search for an answer to this question, but never managed to find any.

What is the standard ratio for allocating size to the Java GC spaces when I define a maximum heap size?

That is: if I start my JVM with, say 4Gb of heap, how much is allocated to the Eden? How much to survivors? How much to tenured?

Also, does that ratio change for different VM vendors?

Thanks in advance

回答1:

This is JVM-specific. On Oracle's JVMs, you can find out the heap sizes by using -XX:+PrintCommandLineFlags -XX:+PrintGCDetails:

$ java -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -version
-XX:InitialHeapSize=78820288 -XX:MaxHeapSize=1261124608 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:+UseCompressedOops -XX:+UseParallelGC 
java version "1.7.0_09"
Java(TM) SE Runtime Environment (build 1.7.0_09-b05)
Java HotSpot(TM) 64-Bit Server VM (build 23.5-b02, mixed mode)
Heap
 PSYoungGen      total 22464K, used 770K [0x000000013e820000, 0x0000000140130000, 0x0000000157970000)
  eden space 19264K, 4% used [0x000000013e820000,0x000000013e8e0b08,0x000000013faf0000)
  from space 3200K, 0% used [0x000000013fe10000,0x000000013fe10000,0x0000000140130000)
  to   space 3200K, 0% used [0x000000013faf0000,0x000000013faf0000,0x000000013fe10000)
 ParOldGen       total 51328K, used 0K [0x000000010c570000, 0x000000010f790000, 0x000000013e820000)
  object space 51328K, 0% used [0x000000010c570000,0x000000010c570000,0x000000010f790000)
 PSPermGen       total 21248K, used 2165K [0x0000000107370000, 0x0000000108830000, 0x000000010c570000)
  object space 21248K, 10% used [0x0000000107370000,0x000000010758d6a8,0x0000000108830000)

From this, you can easily work out the ratios.