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matheusmoreira • today at 12:21 AM • 2 replies • view on HN

> Operating systems can grow the amount of real memory allocated to a thread, but never shrink it.

Operating systems can shrink the memory usage of a stack.

  madvise(page, size, MADV_DONTNEED);
Leaves the memory mapping intact but the kernel frees underlying resources. Subsequent accesses get either new zero pages or the original file's pages.

Linux also supports mremap, which is essentially a kernel version of realloc. Supports growing and shrinking memory mappings.

  stack = mremap(stack, old_size, old_size / 2, MREMAP_MAYMOVE, 0);
Whether existing systems make use of this is another matter entirely. My language uses mremap for growth and shrinkage of stacks. C programs can't do it because pointers to stack allocated objects may exist.

Replies

dwattttt • today at 12:07 PM

> C programs can't do it because pointers to stack allocated objects may exist.

They sure shouldn't exist to the unused region of the stack though; if they do, that's a bug (because anything could claim that memory now). You should be free and clear to release stack pages past your current stack pointer.

simonask • today at 6:42 AM

Stack memory is never unmapped until the thread terminates as far as I know. I don’t know of any kernel that does this, for precisely the reason you arrive at by the very last sentence.