Why Are Stacks Limited in Size While Heaps Seem Unlimited?

Stack Memory Size Limitations: Technical Considerations

The stack, a fundamental data structure in computing, is typically characterized by its limited size compared to the heap. While the heap offers seemingly limitless memory allocation, the stack's size is typically constrained to around 1 MB. This raises the question: what technical factors contribute to this disparity?

According to one hypothesis, the restricted size of the stack stems from its inherent management complexity compared to the heap. The stack requires continuous memory locations for its operation. Unlike the heap, it cannot allocate memory randomly as needed, but rather must reserve virtual addresses for this purpose. The larger the virtual address space reserved, the fewer threads can be created.

For instance, in a 32-bit application with a 2GB virtual address space, a 2MB stack size (as in pthreads) limits the maximum number of threads to 1024. This limitation can become problematic for applications like web servers that require a large number of threads. Increasing the stack size to 100MB, while not allocating the full amount immediately, would further reduce the thread limit to around 20. Such constraints can hinder even simple GUI applications.

Curiously, the same limitation persists on 64-bit platforms, where there is ample virtual address space. To explain this, some speculate that established "stack best practices" have influenced the lack of support for large stacks on 64-bit systems. Developers are accustomed to allocating large objects on the heap and manually increasing the stack size when necessary. As a result, the perceived need for "huge" stack support has not emerged on 64-bit architectures.

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