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I understand that each thread has its own stack. Primitive types and references are kept on the stack, and that no object is kept on the stack.
My questions are:
Xms and -Xmx)
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Stacks are temporary memory address spaces used to hold arguments and automatic variables during invocation of a subprogram or function reference. In general, the default main stack size is 8 megabytes.
The Java stack size is the size limit of each Java thread in the Java Virtual Machine (JVM) that runs the monitoring Model Repository Service. The default value is 512K.
Because all threads in a process share the same address space, they have to divide it between them. And after the operating system has taken its part, there is "only" 2-3 GB left for an application. And that size is the limit for both the physical and the virtual memory, because there just aren't any more addresses.
Java Heap space is used by java runtime to allocate memory to Objects and JRE classes. Whenever we create an object, it's always created in the Heap space. Garbage Collection runs on the heap memory to free the memory used by objects that don't have any reference.
You can use a VM option named ss to adjust the maximum stack size. A VM option is usually passed using -X{option}. So you can use java -Xss1M to set the maximum of stack size to 1M.
Each thread has at least one stack. Some Java Virtual Machines (JVM) put Java stack (Java method calls) and native stack (Native method calls in VM) into one stack, and perform stack unwinding using a "Managed to Native Frame", known as M2nFrame. Some JVMs keep two stacks separately. The Xss set the size of the Java Stack in most cases.
For many JVMs, they put different default values for stack size on different platforms.
When a method call occurs, a new stack frame will be created on the stack of that thread. The stack will contain local variables, parameters, return address, etc. In Java, you can never put an object on stack, only object reference can be stored on stack. Since array is also an object in Java, arrays are also not stored on stack. So, if you reduce the amount of your local primitive variables, parameters by grouping them into objects, you can reduce the space on stack. Actually, the fact that we cannot explicitly put objects on Java stack affects the performance some time (cache miss).
As I said before, different VMs are different, and may change over versions. See here.
Garbage collections in Java is a hot topic. Garbage collection aims to collect unreachable objects in the heap. So that needs a definition of 'reachable.' Everything on the stack constitutes part of the root set references in GC. Everything that is reachable from every stack of every thread should be considered as live. There are some other root set references, like Thread objects and some class objects.
This is only a very vague use of stack on GC. Currently most JVMs are using a generational GC. This article gives brief introduction about Java GC. And recently I read a very good article talking about the GC on .NET platform. The GC on Oracle JVM is quite similar so I think that might also help you.
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