Question

When you allocate memory on the heap, the only limit is free RAM (or virtual memory). It makes GB of memory.

So why is stack size so limited (around 1 MB)? What technical reason prevents you to create really big objects on the stack?

Update: My intent might not be clear, I do not want to allocate huge objects on the stack and I do not need a bigger stack. This question is just pure curiosity!

Answer 1

My intuition is the following. The stack is not as easy to manage as the heap. The stack need to be stored in continuous memory locations. This means that you cannot randomly allocate the stack as needed, but you need to at least reserve virtual addresses for that purpose. The larger the size of the reserved virtual address space, the fewer threads you can create.

For example, a 32-bit application generally has a virtual address space of 2GB. This means that if the stack size is 2MB (as default in pthreads), then you can create a maximum of 1024 threads. This can be small for applications such as web servers. Increasing the stack size to, say, 100MB (i.e., you reserve 100MB, but do not necessarily allocated 100MB to the stack immediately), would limit the number of threads to about 20, which can be limiting even for simple GUI applications.

A interesting question is, why do we still have this limit on 64-bit platforms. I do not know the answer, but I assume that people are already used to some “stack best practices”: be careful to allocate huge objects on the heap and, if needed, manually increase the stack size. Therefore, nobody found it useful to add “huge” stack support on 64-bit platforms.