CODEWITHSUNDEEP
java
Evans Y.
Evans Y.

Reputation: 4429

Why the JVM's Runtime max memory is smaller than the value specified in command line -Xmx?

I start my program by specifying the max memory to 128MB

java -Xmx128M ...

Then I connect to the jvm instance with jconsole. In the "VM Summary" tab, I found that: 

Maximum heap size: 127,808 kbytes

This value is smaller than the one I specified in command line. Can any one give me some tips on this?

Upvotes: 0

Views: 977

Answers (3)

Stephane Grenier
Stephane Grenier

Reputation: 15925

To quote the following Java bug report:

Not a bug. The interpretation of the -Xmx flag is VM-dependent. Some VMs, including HotSpot, enforce a lower bound on the effective value of this option. The CCC proposal should not have mentioned the -Xmx flag in this way.

Upvotes: 0

FrVaBe
FrVaBe

Reputation: 49351

I checked the value that is reported by Java Visual VM. It is exactly what I configured as JVM argument:

Example

JVM argument: 1024m
Heap Max: 1.073.741.824 B

So I guess jconsole has a special kind of calculation or Java Visual VM adjusts the value to the configured JVM argument - who knows?

Upvotes: 2

Peter Lawrey
Peter Lawrey

Reputation: 533790

I have found the exact memory size to be slightly smaller. How much smaller apepars to vary based on the version of Java and the OS. I suspect there is some implementation reasons why its is not precisely what you asked for.

BTW: This is only the maximum of the heap size. There are many other memory areas e.g. thread stack, direct memory, perm gen, shared libraries, native resources and memory mapped files which are not included so I wouldn't woory too much about it i.e. as there is a good margin of error between this value and the maximum memory the application will use.

BTW2: A maximum of 128m is rather small these days. Is there a good reason for it to be so small?

Upvotes: 1

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