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I have set the default memory limit of Java Virtual Machine while running Java Application like this...

java -mx128m ClassName

I Know this will set maximum memory allocation pool to 128MB, but I don't know what the benefit is, of specifying this memory limit of JVM?

Please enlighten me in this issue...

Peter
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Saravanan
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5 Answers5

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On Sun's 1.6 JVM, on a server-class machine (meaning one with 2 CPUs and at least 2GB of physical memory) the default maximum heap size is the smaller of 1/4th of the physical memory or 1GB. Using -Xmx lets you change that.

Why would you want to limit the amount of memory Java uses? Two reasons.

Firstly, Java's automatic memory management tends to grab as much memory from the operating system as possible, and then manage it for the benefit of the program. If you are running other programs on the same machine as your Java program, then it will grab more than its fair share of memory, putting pressure on them. If you are running multiple copies of your Java program, they will compete with each other, and you may end up with some instances being starved of memory. Putting a cap on the heap size lets you manage this - if you have 32 GB of RAM, and are running four processes, you can limit each heap to about 8 GB (a bit less would be better), and be confident they will all get the right amount of memory.

Secondly (another aspect of the first, really), if a process grabs more memory than the operating system can supply from physical memory, it uses virtual memory, which gets paged out to disk. This is very slow. Java can reduce its memory usage by making its garbage collector work harder. This is also slow - but not as slow as going to disk. So, you can limit the heap size to avoid the Java process being paged, and so improve performance.

Tom Anderson
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There will be a default heap size limit defined for the JVM. This setting lets you override it, usually so that you can specify that you want more memory to be allocated to the java process.

krock
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  • what is the default memory limit of JVM? – Saravanan Sep 14 '11 at 07:23
  • @Saravanan, the default is jvm implementation specific, and a default may even "be chosen at runtime based on system configuration" on some JVMs. The default on my machine is 64MB. – krock Sep 14 '11 at 07:31
  • ,Thanks a lot. how to check it? – Saravanan Sep 14 '11 at 08:03
  • @Saravanan, at runtime you can run `Runtime.getRuntime().maxMemory();` to find out what setting the jvm is using, otherwise consult the documentation for your jvm, I ran `man java` to look up the default value on my machine. – krock Sep 14 '11 at 08:14
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  1. This sets the maximum Heap Size. The total VM might be larger
  2. There is always a limit because this parameter has a default value (at least for the Oracle/Sun VM)
  3. So the benefit might either be: you can give the memory to the app that it actually needs in order to work (efficiently) or if you come from the other direction: (somewhat) limit the maximum memory used in order to manage the distribution of resources among different applications on one machine.
Jens Schauder
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There already has been a question about java and memory SO: Java memory explained

A very nice article about Java memory is found here. It gives an overview of the memory, how it is used, how it is cleaned and how it can be measured.

The defaults of the memory are (prior java 6):

-Xms size in bytes Sets the initial size of the Java heap. The default size is 2097152 (2MB). The values must be a multiple of, and greater than, 1024 bytes (1KB). (The -server flag increases the default size to 32M.)

-Xmn size in bytes Sets the initial Java heap size for the Eden generation. The default value is 640K. (The -server flag increases the default size to 2M.)

-Xmx size in bytes Sets the maximum size to which the Java heap can grow. The default size is 64M. (The -server flag increases the default size to 128M.) The maximum heap limit is about 2 GB (2048MB).

Another source (here) states that in Java 6 the default heap size depends on the amount of system memory.

Community
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Oosterman
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I assume this should help avoid high memory consumption (due to bugs or due to many allocations and deallocations). You would use this if you design for a low-memory system (such as an old computer with little amounts of RAM, mobile phones, etc.).

Alternatively, use this to increase the default memory limit, if it is not enough for you and you are getting OutOfMemoryExceptions for normal behavior.

Eran Zimmerman Gonen
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