[Scaevolus]

Not if they're done as split/segmented stacks (http://gcc.gnu.org/wiki/SplitStacks)-- basically, you have a collection of 4KB stack pages for each thread instead of one large up-front allocation, and you grow it as needed. It costs a few instructions per function entry/exit, but overall cost is negligible and it allows you to run thousands of coroutines without issues.

[DanWaterworth]

If you allocate the stacks contiguously using mmap then memory is only used as it is accessed. That's not the problem. The problem is that 4000 concurrent non-trivial threads is a resource hog no matter how the stacks are allocated.