Maximum heap size for 32 bit or 64 bit JVM looks easy to determine by looking at addressable memory space like 2^32 (4GB) for 32 bit JVM and 2^64 for 64 bit JVM. Confusion starts here because you can not really set 4GB as maximum heap size for 32 bit JVM using -Xmx JVM heap options. You will get could not create the Java virtual machine Invalid maximum heap size: -Xmx error. There could be many different reason why maximum heap space for JVM is less than there theoretical limit and vary from one operating system to other e.g. different in Windows, Linux and Solaris. I have seen couple of comments on my post 10 points on Java Heap Space regarding what is maximum heap space for Java or 32 bit JVM or 64 bit JVM and why Windows allows only upto 1.6G memory as maximum heap space etc. In this Java article, I have collected some of the frequently asked questions around maximum heap space on both 32 and 64 bit JVM and tried to explain them.
FAQ Maximum Java Heap Space on 32 and 64 bit JVM
Here is list of some confusions I have seen on Java programmers regarding maximum heap space of 32 and 64 bit Java Virtual Machines :
- What is maximum heap size for 32 bit JVM? 2GB or 4GB?
- Why my JVM not able to start on windows when maximum heap space around 1600M?
- Why Linux or Solaris allow more maximum heap size than windows for same, 32 bit JVM?
- Can we set more than 4GB as maximum heap size for 32 bit JVM running on 64 bit or x64 operating system?
- What is maximum heap size for 64 bit or x64 JVM, Is it 8GB or 16GB?
- Can I specify more than 1GB as heap space if physical memory is less than 1GB?
If you also have similar confusion on JVM maximum heap space no matter whether its for your own Java application or any Java web or application server like Tomcat, JBoss or WebLogic, This discussion applies to all of them.
What is maximum heap size for 32 bit JVM? 2GB or 4GB?
This confusion comes because of sign bit, many programmer think in terms of signed integer and they think maximum addressable memory (size of address bus) for 32 bit architecture is 2^32-1 or 2GB and this confusion is supported by fact that you can not provide maximum heap space as 2GB on windows machine. But this is wrong. Memory is nothing to do with signed or unsigned bit as there is no negative memory address. So theoretical limit for maximum heap size on 32 bit JVM is 4GB and for 64 bit JVM its 2^64.
Why JVM not able to start on Windows XP when maximum heap space around 1600M?
This problem is most obvious on Windows platform like Windows XP, which tries to allocate a contiguous chunk of memory as requested by -Xmx JVM parameters. Windows reserves some space for his own and seems also allocate memory around half of memory address bar, which consequently reduces contiguous memory space somewhere less than 2GB, around 1500 to 1600M and when you give more than this size, JVM throws error as.
Could not create the Java virtual machine.
Invalid initial heap size: -Xms1.5G
Remember, this limit on heap space is due to windows operating system's own behavior. You can set maximum heap space, more than this size in Linux or Solaris. Though maximum heap size for 32 bit or 64 bit JVM will always be less than theoretical limit of addressable memory. By the way you can get this error due to many reasons, see How to fix for more details.
Why Linux or Solaris allow more maximum heap size than windows for same, 32 bit JVM?
This point is also related to second. Though there could be multiple reasons for that but I think It could be because of Windows trying to allocate contiguous chunk of memory as Java heap space. Happy to hear your opinion on this.
Can we set more than 4GB as maximum heap size for 32 bit JVM running on 64 bit or x64 operating system?
This is a tricky question as you are running 32 bit JVM on x64 server. In my opinion you can set upto 4GB for 32 bit JVM but not more than that. Though x64 Servers has more memory for his needs and since every process can have upto 2^64 bit it may look perfectly OK for 32 bit JVM to accept 4GB as maximum heap size. In practice, I have tried both Linux and Solaris servers setting maximum heap size as 4G but it didn’t accept. Solaris goes more close to 4GB by allowing upto 3.6G (approx).
What is maximum heap size for 64 bit or x64 JVM, Is it 8GB or 16GB?
This question mostly arise because of available physical memory on machine. As no system currently have 2^64 bit of physical memory or RAM and often high end servers has memory around 8G, 16GB or 32GB. Theoretical maximum memory for x64 machines is 2^64 bit but again its depend on how much your operating systems allows.I read some where that Windows allowed maximum of 32GB for 64 bit JVM.
Can I specify more than 1GB as heap space if physical memory is less than 1GB ?
Theoretically yes, because operating system can use virtual memory and swap pages between physical memory and virtual memory, when there is no room in physical memory. Practically, if you are running on windows than it depends how far you can go, I have run Java program with -Xmx1124M even though my machine has less than 1GB RAM.
That’s all on what is maximum Java heap space for 32 bit and 64 bit JVM. As you see maximum heap size depends upon host operating system. Solaris and Linux provides more heap space than windows and that could be one of the many reason that Java Server application mostly run on UNIX based systems. Let me know what’s your thought and experience on maximum Java heap space for x86 and x64 JVM running on both x86 and x64 machines.
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