2. actually return memory to the OS. This is, in general, not possible. In *nix, once a process is allocated pages, there is no mechanism for giving them back. The only exception to this is memory added via mmap. (...) mmap, however, is typically only used for very specific things, like shared-lib access

But this seems to be just what is happening (i.e. mmap/munmap): 

  PID    SZ    VSZ
 5645   604  11064
  PID    SZ    VSZ
 5645  4516  14976
  PID    SZ    VSZ
 5645  2560  13020

[download]

(This is the output I get when running Joost's snippet — even on an old Linux box with a rather rusty 2.6.16 kernel.)

The last memory printout is definately about 2M smaller (the PV/string part of that scalar variable) than the previous one...

And strace shows — when I replace the system("/bin/ps",...) with a simple print "---\n" (to leave a marker in the output): 

$ strace ./746953.pl
(...)
brk(0x66a000)                           = 0x66a000
read(4, "#!/usr/bin/perl\n\nsub ps {\n    #s"..., 8192) = 1003
fcntl(4, F_SETFD, FD_CLOEXEC)           = 0
write(1, "---\n", 4---
)                    = 4
mmap(NULL, 2002944, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -
+1, 0) = 0x2ba8bbdd5000
mmap(NULL, 2002944, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -
+1, 0) = 0x2ba8bbfbe000
write(1, "---\n", 4---
)                    = 4
munmap(0x2ba8bbfbe000, 2002944)         = 0
write(1, "---\n", 4---
)                    = 4
lseek(4, 196, SEEK_SET)                 = 196
lseek(4, 0, SEEK_CUR)                   = 196
close(4)                                = 0
exit_group(0)

[download]

In reply to Re^2: demonstrate that perl can give back memory to the OS by almut
in thread demonstrate that perl can give back memory to the OS by perl5ever

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