Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64)

[Roy Smith]

In article <khg1v3$l7q$1 at reader2.panix.com>,
 Grant Edwards <invalid at invalid.invalid> wrote:

> In Unix there is no way to release heap memory (which is what you're
> talking about) back to the OS except for terminating the process.

That's not quite true.  The man page for BRK(2) (at least on the Linux 
box I happen to have handy) says:

"brk() and sbrk() change the location of the program break, which 
defines the  end  of  the process's data segment (i.e., the program 
break is the first location after the end of the uninitialized data 
segment).  Increasing the program break has the  effect  of  allocating 
memory to the process; decreasing the break deallocates memory."

So, in theory, it's possible.  I just ran this C program:

#include <unistd.h>
#include <time.h>

int main(int argc, char** argv) {
    struct timespec t;
    t.tv_sec = 10;
    t.tv_nsec = 0;

    nanosleep(&t, NULL);
    sbrk(500 * 1024 * 1024);

    nanosleep(&t, NULL);
    sbrk(-500 * 1024 * 1024);

    nanosleep(&t, NULL);
}

while watching the process with ps.  I could see the process appear and 
for the first 10 seconds it had a VSZ of 4156.  Then, for the next 10 
seconds, the VSZ was 516156, then it went back down to 4156.

$ while sleep 1; do ps augx | grep a.out | grep -v grep; done
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0 516156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0 516156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0 516156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0 516156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0 516156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0 516156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0 516156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0 516156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0 516156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out
roy       6657  0.0  0.0   4156   356 pts/10   S+   19:56   0:00 ./a.out

In practice, unless you go to extraordinary lengths (i.e. avoid almost 
all of the standard C library), the break is going to be managed by 
malloc(), and malloc() provides no way to give memory back (insert 
handwave here about mmap, and another handwave about various versions of 
malloc).  But, that's due to the way malloc() manages its arena, not 
anything fundamental about how Unix works.