With POSIX threads the stack size defaults to something like 1MB or 2MB depending on the platform, but it's not allocated up front -- the stack grows as needed

With POSIX threads the stack size defaults to something like 1MB or 2MB depending on the platform, but it's not allocated up front -- the stack grows as needed up to that maximum. The main difference then between allocating stack chunks on the heap as needed, and stacks grown by the virtual memory subsystem, has to do with virtual memory management matters. If you can use huge pages for your heap, then allocating stack chunks on the heap will be cheaper than traditional stacks.