A while back, I did a simple test of mallocing a 4 MB chunk
of memory, then freeing it. The memory was returned to the
OS on Windows and Linux -- but not on every other of the 12 different
platforms I ran the test on.
Whether the memory is returned to the OS or not depends on the
implementation of the user-level malloc function.
Most implementations of malloc simply call sbrk(2) to increase
the address space of the process. Since this call is expensive,
when you call free, they typically do not return the memory to
the OS, but keep it to reuse next time you call malloc.
glibc 2.x based Linux systems use ptmalloc, which is based on
Doug Lea's malloc. This version of malloc is able to return
memory to the OS because it uses mmap(2), rather than sbrk,
when the chunk of memory requested is larger than some
threshold value (typically 128K). MSVC CRT similarly uses
a "small block" and "large block" heap heuristic -- but uses
Win32 heaps, rather than sbrk/mmap, under the covers.
So, if you were desperate to have memory returned to the OS,
you could go to the bother of building a custom perl which used
a different implementation of malloc.
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