Monday, December 30, 2013

JRockit: What's the Total Memory Footprint of a Java Process?

In [1], we have shown a case of performance tuning by sizing JRockit's Thread Local Area (TLA). For both test runs—TLA default and TLA tuned, they have been given the same heap size (i.e, 2g). However, in the conclusion, I have said:
Better performance is achieved by reducing pause time % in GC andTotal CPU % at the expense of total memory footprint (-1.6%).
In this article, we will clarify what the following phrase:
at the expense of total memory footprint (-1.6%)
means.

Java Heap vs Native Memory


As described in [2], total memory footprint of a Java process includes not only Java Heap, but also Native Memory. For example, you can use print_memusage to analyze the memory (including native memory) allocated by a Java process:

> ./jrcmd 26413 print_memusage >JR_print_memusage.txt
>cat JR_print_memusage.txt


26413:
Total mapped                  4897980KB           (reserved=1384484KB)
-              Java heap      2097152KB           (reserved=0KB)
-              GC tables        70156KB
-          Thread stacks        48324KB           (#threads=141)
-          Compiled code      1048576KB           (used=41160KB)
-               Internal         1480KB
-                     OS       401856KB
-                  Other       760932KB
-            Classblocks        27136KB           (malloced=26821KB #58548)
-        Java class data       441344KB           (malloced=439083KB #271063 in 58548 classes)
- Native memory tracking         1024KB           (malloced=242KB #10)

Native Memory


Internal JVM memory management is, to a large extent, kept off the Java heap and allocated natively in the operating system, through system calls like malloc. This non-heap system memory allocated by the JVM is referred to as native memory.

For JRockit, you can constraint the amount of memory allocated to the Java heap, but not the native memory, in a Java process.[3] As shown above, process 26413 has been allocated 4897980KB (i.e., VSZ) in the virtual address space. However, this VSZ value is not very useful. What counts is the one reported by Resident Set Size (RSS; or physically resident memory). To find out the RSS of a Java process, you can do:

>ps -o pid,uid,state,rss,vsz,minflt,majflt,args -p 26413 >ps_26413.tmp
>cat ps_26413.tmp

PID   UID S   RSS    VSZ MINFLT MAJFLT COMMAND
26413 60000 S 3295216 4889792 15194852 1 /scratch/user1/JVMs/jdk-jr/bin/java ...

For the process 26413, its total memory footprint (or RSS) is 3295216 KB and this is the KPI we have quoted in our benchmark comparison.

References

  1. JRockit: A Case Study of Thread Local Area (TLA) Tuning
  2. Why is my JVM process larger than max heap size?
  3. How to Debug Native OutOfMemory in JRockit (Xml and More)

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