I know the default Java heap memory is 128 MB. Since it is the default, I want to know whether this memory get automatically changed according to the RAM size. For an example, for a machine with 128 MB RAM, heap memory 128 is too much and it should be automatically changed. Because if an application use all of 128 heap, PC will end up in a trouble.
please help.
In Java 1.6 update 18 (and later), if unspecified, the default heap-size in a client JVM follows these rules:
The default maximum heap size is half of the physical memory up to a
physical memory size of 192 megabytes and otherwise one fourth of the
physical memory up to a physical memory size of 1 gigabyte.
For
example, if your machine has 128 megabytes of physical memory, then
the maximum heap size is 64 megabytes, and greater than or equal to 1
gigabyte of physical memory results in a maximum heap size of 256
megabytes.
Taken from the 1.6.0_18 update notes
In previous releases of Java heap size was NOT variable by default.
No, it won't be automatically changed unless you ask for it on startup using the -Xmx option.
You can't get more than 2 GB on a 32 bit machine in any case. If you need more than that, you'll have to get a 64 bit operating system with lots of RAM.
You can specify JVM heap size by using aptions -Xmx and -Xms
-Xmx - Max Size.
-Xms - Min Size.
You specify like
-Xmx64m -Xms32m (for 64 and 32 MB)
No. It is not automatically changed. Look here for more on HotSpot JVM memory options .
More Here on handling Java OutOfMemoryError.
Related
I am using a 64 bit Windows 10 system with 32Gb of RAM. One of my 32 bit java application requires a lot of memory so I decided to change the minimum heap size to -Xms4096m at the runtime parameter field in the java configuration dialogue.
However, I soon realised that it would not do anything because apparently my system's maximum heap size is only 247.50mb (which is way too small for a 64 bit system). I am using a 32 bit java so I am expecting a max heap size of around 4GB. Can anyone tell me what determines the maximum heap size and how do I change it?
UPDATED: I understand that the maximum I can allocate is 2GB for a 32 bit system. So my question now is how come for my system, its only 247.50mb?
BELOW ARE MY Java Settings:
I am running java process through main class on java 8. I have not specified anywhere min(Xms) and max(Xmx) heap size. But when i check through visualVM it's
4267704320(i.e. 4.26 GB) which is the default max heap size for a given process(confirmed through windows command also which is
-XX:+PrintFlagsFinal -version 2>&1 | findstr MaxHeapSize. similarly on linux box too).
My question is if my process(on linux machine) requires more 5 GB(i have 30 GB RAM), will my process be allocated 5 GB memory when i have not specified
any explicit heap size(Xms and Xmx) parameters.
Given your the available RAM on your machine, if you are running a 64-bit JVM in server mode, then yes, the heap size will be able to go up to approximately 7.5 GB.
Documentation (I highlighted the relevant parts):
Client JVM Default Initial and Maximum Heap Sizes
The default maximum heap size is half of the physical memory up to a physical memory size of 192 megabytes (MB) and otherwise one fourth of the physical memory up to a physical memory size of 1 gigabyte (GB).
Server JVM Default Initial and Maximum Heap Sizes
The default initial and maximum heap sizes work similarly on the server JVM as it does on the client JVM, except that the default values can go higher. On 32-bit JVMs, the default maximum heap size can be up to 1 GB if there is 4 GB or more of physical memory. On 64-bit JVMs, the default maximum heap size can be up to 32 GB if there is 128 GB or more of physical memory. You can always set a higher or lower initial and maximum heap by specifying those values directly; see the next section.
Again, assuming a 64-bit JVM running in server mode, the default max heap size according to the documentation will be one fourth of your total RAM. So approximately 7.5 GB in your case (1/4 of 30 GB).
If running a 32-bit JVM in server mode, you'll be capped at 1 GB. And in client mode, your max will be 256 MB.
Heap is part of RAM.But still there is a limit that we can not define heap size beyond some percentage of RAM.For example:- i have 32 bit winodws xp OS and 4
GB RAM. But i can not declare the heap size more than 1600 MB. My question here why we can not declare heap size to some large value say 3GB which
is lower than my 4GB RAM(as in 32 Bit can ustilize up to 4gb of RAM)? This is true for single process.
I mean i can start two tomcat or any other java process
allocating 1600MB heap size to each but i can not allocate 3200MB to single process.what is the reason that behind that?
32-bit windows only allows 2GB of address space to a single process (without special extensions enabled.) The OS keeps the other 2GB for itself. Then on top of that Heap is not all of the memory that JVM needs. There is permgen space, and the memory that the code of the JVM itself uses.
If its not 64 bit 4GB cannot be allocated.
What's the maximum amount of heap space that one can allocate for java on a 64-bit platform? Is it unlimited?
Theoretically 264, but there might be limitations (obviously)
According to this FAQ it's only limited by memory and swap space on the local system:On 64-bit VMs, you have 64 bits of addressability to work with resulting in a maximum Java heap size limited only by the amount of physical memory and swap space your system provides.
See also Why can't I get a larger heap with the 32-bit JVM?
Also keep in mind you need to set the max heap via command line. Without an -Xmx command. without it Java uses 64mb + 30% = 83.2mb as a default max heap on 64 bit machines according the same FAQ.
java -Xmx1000g myClass
works fine on my machine. But it dosn't seem to yet support the 't' modifier so you can't specify max memory in terabytes yet :)
If you could make every atom in the universe into a byte of RAM, you could allocate it in a 64 bit address space.
Actually, that's a slight exaggeration.
There are 10^80 atoms in the universe (according to WolframAlpha), and 2^64 bytes of address space in a 64 bit system, so you'd only be able to address 1 out of every 5x10^60 atoms. But if you have 18 qintillion bytes of RAM, you'd probably need a couple of quantum black holes to power it with.
This probably depends on the system your VM is running in. If you are running a AMD x64 architecture the address space of currently shipped processors uses 48 Bits, not 64. This results in a theoretical maximum of roughly 256 TB. (See http://en.wikipedia.org/wiki/X86-64)
I am not a specialist in VMs, but any modern OS typically will give as much memory as there is physical RAM plus available virtual memory. Probably thats what the VM will pass to your application depending on its configuration.
With recent VMs from Sun, the practical heap limit size is usually 512 times the available physical and/or virtual memory. Even if the theoretical limit is much higher, the VM will allocate 1 byte for maangement purposes for each 512 bytes of heap memory on startup, so 1TB of heap will immediately require 2GB for the memory management.
In theory its between 2^63 and 2^64 bytes.
In practice it is limited by the amount of physical memory and swap space available on your machine. And the physical memory is in turn limited by your chipset (i.e. the number of address pins on the physical memory address bus) and motherboard (i.e. the number and size of the DIMM sockets).
I run a Java code with the following command:
$ java -Xms4G -Xmx4G myjavacode
My cpu's RAM capacity is 6GB.
However it always fail to execute giving me this
error message:
Invalid initial heap size: -Xms5G
The specified size exceeds the maximum representable size.
Could not create the Java virtual machine
Is there any way to set up Java option so that we can
execute the code?
You've exceeded the maximum heap size of your JVM. This is both JVM and OS dependent. In most 32-bit systems the maximum value will be 2Gb, regardless of the physical memory available.
By default Java will run in 32 bit mode. Be sure to give it the -d64 option to put it into 64 bit mode. Once in 64-bit mode, you shouldn't have any trouble allocating a 6GB JVM.
Actually, the maximum memory size on 32-bit systems can vary, being anything up to 4 GB, but 2 GB is a common value. It's often possible to re-link your kernel to increase this to 3 or 3.5 GB. The issue, of course, is that you just don't have the address space to map more memory. Have you tried a 64-bit machine?
Also, remember to set your ulimit higher before you do this.