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How is the default java heap size determined?

Im curious what happens if I do not specify a -Xms for initial JVM heap size with Java?

Currently I have -Xms256m and -Xmx512m but this gives me an error on my server :

Failed to create JVM, return code is:-1

I believe the JVM is having trouble getting 256MB of contiguous memory with a 256m starting point. Prior to today we only had a -Xmx256m setting but had some processes that were running out of memory.

Can the JVM function if I remove the -Xms setting and just leave the -Xmx512m'? What does the heap start with?

How does it acquire contiguous memory if no starting heap is declared?

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  • @TedHopp - But if I specify the max, how does the system ensure it gets contiguous memory? Commented Aug 15, 2012 at 15:08

3 Answers 3

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To answer your first question. Please look at Ted Hopp's comment. How is the default java heap size determined?

Secondly you don't need to specify and minimum and a maximum heap size. You are also inputting them incorrectly.

It should be -Xms256m -Xmx512m

Initial heap size will be 4Mbyte with a max heap of 64Mbyte on client class machines.

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5 Comments

But if I specify the max, how does the system ensure it gets contiguous memory if I allow the system to default with the min?
Is there a reason you need a contiguous memory region?
The JVM requires it. We are still using Oracle/Sun JDK (1.5 I believe) so it requires contiguous memory. Our documentation states But some times XML Publisher kernel will not start if JVM could not get contiguous block of memory.
I see. Well have you tried without setting the initial heap size? It should (or at least I would hope it should) do some talking with the OS to request the address space as the requirement grows. If you are on a machine with limited memory, the only option would be to go 64bit java as getting contiguous memory region is easier in that address space.
I can try that, Im searching for info on how the JVM does that though before I do. JRockit JVM doesnt require contiguous memory too, but I do not want to swap the JVMs on all my servers if I can help it now. Appreciate the input
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Default Initial size:

Larger of 1/64th of the System's physical memory on the machine or some reasonable minimum. Before J2SE 5.0, the default initial heap size was a reasonable minimum, which varies by platform.

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But if I specify the max, how does the system ensure it gets contiguous memory?
@ProfessionalAmateur It doesn't have to. That's what virtual memory is for, among other things.
@EJP - But my kernel does required it for some odd reason. But some times XML Publisher kernel will not start if JVM could not get contiguous block of memory. Maybe my kernel requests it from the JVM and the JVM itself doesnt care.
@ProfessionalAmateur What kernel are you referring to? Some application? Kernels don't request things from the JVM: it's the other way around. I think you've misunderstood something here. The last non-virtual-memory system I worked on was in 1993.
@EJP - This is a BI Publisher kernel for an Oracle product we have on a large enterprise system. Essentially a mail merge process that takes XML values and merges them with a report template to produce reports/pdfs/excel/etc... Im guessing that this process needs contiguous memory to interact with the JVM and not the JVM itself at this point.
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Don't put the =:

-Xms256m -Xmx512m

I think that the default has historically 2 MB, but it depends on your version of Java and your platform. I'll try to dig up the docs.

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You're right, we did not typo in my post, Ill fix it.

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