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

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On 2013-03-09, Roy Smith <roy@panix.com> wrote:
> In article <khg1v3$l7q$1@reader2.panix.com>,
>  Grant Edwards <invalid@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.

Well spotted.  I would have bet money (OK, not a lot), that sbrk()
only accepted positive increment values.  I seem to have conflated
what sbrk() will accept and the fact that the C library's malloc/free
calls never call sbrk() with a nagative number.

[... nicely done demonstraction of negative sbrk() parameter usage ...]

> 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.

Indeed.

What I should have said was that there's no way to return to the OS
memory obtained via calls to malloc() et al, and those are the calls
that "good" C programmers (like the maintainers of CPython) use.

In theory, you could modify the C library so that calls to free()
might return memory to the OS.  Sometimes.  If you're lucky.

The problem is that if there's _one_byte_ of memory in use at the
"end" of the heap region, it doesn't matter if there's an unused 16GB
chunk in the middle -- it can't be returned to the OS.

-- 
Grant Edwards               grant.b.edwards        Yow! Look DEEP into the
                                  at               OPENINGS!!  Do you see any
                              gmail.com            ELVES or EDSELS ... or a
                                                   HIGHBALL?? ...

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sync databse table based on current directory data without losign previous values Νίκος Γκρ33κ <nikos.gr33k@gmail.com> - 2013-03-05 23:45 -0800
  Re: sync databse table based on current directory data without losign previous values Lele Gaifax <lele@metapensiero.it> - 2013-03-06 09:19 +0100
    Re: sync databse table based on current directory data without losign previous values Νίκος Γκρ33κ <nikos.gr33k@gmail.com> - 2013-03-06 00:57 -0800
      Re: sync databse table based on current directory data without losign previous values Lele Gaifax <lele@metapensiero.it> - 2013-03-06 10:24 +0100
        Re: sync databse table based on current directory data without losign previous values Νίκος Γκρ33κ <nikos.gr33k@gmail.com> - 2013-03-06 01:41 -0800
          Re: sync databse table based on current directory data without losign previous values Νίκος Γκρ33κ <nikos.gr33k@gmail.com> - 2013-03-06 01:43 -0800
            Re: sync databse table based on current directory data without losign previous values Bryan Devaney <bryan.devaney@gmail.com> - 2013-03-06 02:15 -0800
            Re: sync databse table based on current directory data without losign previous values Bryan Devaney <bryan.devaney@gmail.com> - 2013-03-06 02:15 -0800
          Re: sync databse table based on current directory data without losign previous values Νίκος Γκρ33κ <nikos.gr33k@gmail.com> - 2013-03-06 01:43 -0800
          Re: sync databse table based on current directory data without losign previous values Lele Gaifax <lele@metapensiero.it> - 2013-03-06 11:27 +0100
          Re: sync databse table based on current directory data without losign previous values Dave Angel <davea@davea.name> - 2013-03-06 08:31 -0500
          Re: sync databse table based on current directory data without losign previous values Lele Gaifax <lele@metapensiero.it> - 2013-03-06 15:16 +0100
        Re: sync databse table based on current directory data without losign previous values Νίκος Γκρ33κ <nikos.gr33k@gmail.com> - 2013-03-06 01:41 -0800
    Re: sync databse table based on current directory data without losign previous values Νίκος Γκρ33κ <nikos.gr33k@gmail.com> - 2013-03-06 00:57 -0800
  Set x to  to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Wong Wah Meng-R32813 <r32813@freescale.com> - 2013-03-06 10:11 +0000
    Re: Set x to  to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Bryan Devaney <bryan.devaney@gmail.com> - 2013-03-06 02:25 -0800
      RE: Set x to  to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Wong Wah Meng-R32813 <r32813@freescale.com> - 2013-03-06 12:31 +0000
      Re: Set x to  to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Dave Angel <davea@davea.name> - 2013-03-06 08:18 -0500
      Re: Set x to  to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Dave Angel <davea@davea.name> - 2013-03-06 08:25 -0500
    Re: Set x to  to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Bryan Devaney <bryan.devaney@gmail.com> - 2013-03-06 02:25 -0800
    Re: Set x to  to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-03-06 23:34 +0000
      RE: Set x to  to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Wong Wah Meng-R32813 <r32813@freescale.com> - 2013-03-07 06:33 +0000
      Re: Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Chris Angelico <rosuav@gmail.com> - 2013-03-07 18:19 +1100
      RE: Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Wong Wah Meng-R32813 <r32813@freescale.com> - 2013-03-08 09:08 +0000
      Re: Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2013-03-08 19:40 -0500
      RE: Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Wong Wah Meng-R32813 <r32813@freescale.com> - 2013-03-09 08:07 +0000
        Re: Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Grant Edwards <invalid@invalid.invalid> - 2013-03-09 19:18 +0000
          Re: Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Roy Smith <roy@panix.com> - 2013-03-09 15:04 -0500
            Re: Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Grant Edwards <invalid@invalid.invalid> - 2013-03-09 20:35 +0000
              Re: Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Roy Smith <roy@panix.com> - 2013-03-09 16:44 -0500
                Re: Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Grant Edwards <invalid@invalid.invalid> - 2013-03-11 14:27 +0000
      Re: Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Dave Angel <davea@davea.name> - 2013-03-09 06:02 -0500
      Re: Set x to to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Isaac To <isaac.to@gmail.com> - 2013-03-09 23:02 +0800
  Re: Set x to  to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Terry Reedy <tjreedy@udel.edu> - 2013-03-06 05:59 -0500
  Re: sync databse table based on current directory data without losign previous values Mark Lawrence <breamoreboy@yahoo.co.uk> - 2013-03-06 11:52 +0000
  RE: Set x to  to None and del x doesn't release memory in python 2.7.1 (HPUX 11.23, ia64) Wong Wah Meng-R32813 <r32813@freescale.com> - 2013-03-06 12:36 +0000
  Re: sync databse table based on current directory data without losign previous values Chris Angelico <rosuav@gmail.com> - 2013-03-07 00:40 +1100
  Re: sync databse table based on current directory data without losign previous values "Michael Ross" <gmx@ross.cx> - 2013-03-06 15:04 +0100
    Re: sync databse table based on current directory data without losign previous values nagia.retsina@gmail.com - 2013-03-06 08:09 -0800
    Re: sync databse table based on current directory data without losign previous values nagia.retsina@gmail.com - 2013-03-06 08:09 -0800

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