Relation between memory host and memory arguments xms and xmx from Java

10
votes

I have the following host with the memory details:

$free -m
             total       used       free     shared    buffers     cached
Mem:          7872       7579        292         17        483       3983
-/+ buffers/cache:       3112       4759 
Swap:         2047         14       2033

I have a java app running with the params -Xms200m -Xmx200m, could someone please explain me why the VCZ is 3800076 and the RSS is 241304 (which is more of the Java params)

from the ps -aux command:

66345     6773  0.2  2.9 3800076 241304 ?      Sl   Apr1  12:06 /apps/myapps/myapp1/java/bin/java -Xms200m -Xmx200m
1
javalinuxmemorymemory-management
Which version of Java is that? - RealSkeptic
Using Java 8 in the host - Weslor

1 Answers

18
votes

Memory used by Java process (as seen by the OS) is not only limited to Java Heap. There are a lot more memory areas that should be also counted:

  • Metaspace (where class metadata resides);
  • Code Cache (storage for JIT-compiled methods and all the generated code);
  • Direct ByteBuffers;
  • Memory-mapped files, including files mapped by JVM, e.g. all JAR files on the classpath;
  • Thread stacks;
  • JVM code itself and all the dynamic libraries loaded by Java Runtime;
  • Other internal JVM structures.

Use NativeMemoryTracking JDK feature to get the detailed breakdown of memory areas used by JVM:

java -XX:NativeMemoryTracking=detail -XX:+UnlockDiagnosticVMOptions -XX:+PrintNMTStatistics