Java heap space does not get increased using -Xmx 1g

Question

I am doing some fancy stuff, where I really need to allocate about 1 GB.

My Computer has 8GB 64 bit System, so that's not a problem.

I use -Xmx 4g to increase my heap space, but sadly enough that doesn't work at all.

I use:

Runtime rt = Runtime.getRuntime();
long totalMem = rt.totalMemory();
long maxMem = rt.maxMemory();
long freeMem = rt.freeMemory();
double megs = 1048576.0;

System.out.println ("Total Memory: " + totalMem + " (" + (totalMem/megs) + " MiB)");
System.out.println ("Max Memory:   " + maxMem + " (" + (maxMem/megs) + " MiB)");
System.out.println ("Free Memory:  " + freeMem + " (" + (freeMem/megs) + " MiB)");

Which shows me:

Total Memory: 259522560 (247.5 MiB
Max Memory:   259522560 (247.5 MiB)
Free Memory:  0 (0 MiB)

I really don't know what I am doing wrong. Does anyone of you have an idea?

Thanks a lot!

Thomas Jungblut · Accepted Answer · 2014-08-30 22:01:49Z

4

It is -Xmx4g, without a space in between.

answered Aug 30, 2014 at 22:01

Thomas Jungblut

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2 Comments

Daniel Over a year ago

-Xmx4g it doesnt make a difference, it still doesnt work

eckes Over a year ago

@Daniel if it does not help, then you are maybe passing the arguments in the wrong way? Can you give us the whole command line you are using? You must set the argument before the class name. If you put it after, it would explain why it did not fail with your superflous blank.

eckes · Accepted Answer · 2014-08-31 00:58:53Z

4

Tell us your whole command line, I suspect you use:

java -jar file.jar -Xmx 4G

or

java -cp file.jar package.Main -Xmx 4G

Both are wrong, you can use one of the following instead:

java -Xmx4G -jar file.jar
java -Xmx4G -cp file.jar package.Main

answered Aug 31, 2014 at 0:58

eckes

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Comments

msduk · Accepted Answer · 2014-08-31 00:35:25Z

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-Xmx4g (with no space, as has been pointed out already) will only set a maxium for the heap size not automatically assign the full amount on startup.

-Xms4g may well do this, as this sets the initial heap size.

Typically people set these as a pair on app startup. For example when launching servers using classes that generate a lot of runtime-generated classes...

-Xms256m -Xmx2048m -XX:MaxPermSize=256m

http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html

answered Aug 31, 2014 at 0:35

msduk

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eckes Over a year ago

maxMemory() is supposed to show the -Xmx setting even when the memory was not commited or used. So I guess the Xms was not the problem here, but it is good to point out.

eckes · Accepted Answer · 2022-10-16 04:30:30Z

The -Xmx option sets the maximum size of the heap. But the JVM typically starts with a smaller heap, and expands it as required.

To tell the JVM the initial size of the heap, use the -Xms option as well; e.g.

   java -Xmx4g -Xms4g ...

tells Java to us a 4g heap, and set that size from the start. Note that this doesn't necessarily cause all of that physical / virtual memory to be allocated to the process immediately. However the heap's space sizes should be set in proportion to the nominal initial size.

(As others have noted, these are atomic options; i.e. don't put a space before 4g.)

If you know that you will need a large part of the requested heap "pretty soon", then setting -Xms avoids (repeated) resizing of the heap during start-up. But if your -Xms value is significantly more than you are likely to need, then you may end up using system resources unnecessarily.

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