Memory management and Process

Is HEAP local to a process? In other words we have stack wh开发者_如何学编程ich is always local to a process and for each process it is seprate. Does the same apply to heap? Also, if HEAP is local, i believe HEAP size should change during runtime as we request more and more memory from CPU, so who puts a top limit on how much memory can be requested?

Heaps are indeed local to the process. Limits are placed by the operating system. Memory can also be limited by the number of bits used for addressing (i.e. 32-bits can only address 2G 4G of memory at a time).

Yes, on modern operating systems there exists a separate heap for each process. There is by the way not just a separate stack for every process, but there is a separate stack for every thread in the process. Thus a process can have quite a number of independent stacks.

But not all operating systems and not all hardware platforms offer this feature. You need a memory management unit (in hardware) for that to work. But desktop computers have that feature since... well... a while back... The 386-CPU? (leave a comment if you know better). You may though find yourself on some kind of micro processor that does not have that feature.

Anyway: The limit to the heap size is mainly limited by the operating system and the hardware. The hardware limits especially due to the limited amount of address space that it allows. For example a 32bit-CPU will not address more than 4GB (2^32). A CPU that features physical address extensions (PAE), which the current CPUs do support, can address up to 64GB, but that's done by the use of segments and one single process will not be able to make use of this feature. It will always see 4GB max.

Additionally the operating system can limit the memory as it sees fit. On Linux you can see and set limits using the ulimit command. If you are running some code not natively, but for example in an interpreter/virtual machine (such as Java, or PHP), then that environment can additionally limit the heap size.

'heap' is local to a proccess, but it is shared among threads, while stack is not, it is per-thread.

Regarding the limit, for example in linux it is set by ulimit (see manpage).

On a modern, preemptively multi-tasking OS, each process gets its own address space. The set of memory pages that it can see are separate from the pages that other processes can see. As a result, yes, each process sees its own stack and heap, because the stack and heap are just areas of memory.

On an older, cooperatively multi-tasking OS, each process shared the same address space, so the heap was effectively shared among all processes.

The heap is defined by the collection of things in it, so the heap size only changes as memory is allocated and freed. This is true regardless to how the OS is managing memory.

The top limit of how much memory can be requested is determined by the memory manager. In a machine without virtual memory, the top limit is simply how much memory is installed in the computer. With virtual memory, the top limit is defined by physical memory plus the size of the swap file on the disk.