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Re: malloc

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In comp.lang.c Kaz Kylheku <kaz@kylheku.com> wrote:
> ["Followup-To:" header set to comp.lang.c.]
> On 2012-02-17, Keith Thompson <kst-u@mib.org> wrote:
<snip>
> > If you call malloc() and it overcommits, it won't crash the
> > program until you access the allocated memory.  (The rationale for
> > overcommitting is that most programs don't actually use most of
> > the memory the memory the allocate.  I find that odd)

> Odd or not, it is borne out empirically. Applications are physically
> smaller than their virtual footprints.

Supposedly, and years ago when application developers where chummy with
the kernel folks.

> It may be the case that C programs that malloc something usually use
> the whole block.
>
> But overcommitting is not implemented at the level of malloc, but
> at the level of a lower level allocator like mmap.
>
> If the system maps a large block to give you a smaller one, that large
> block will not be all used immediately.

No, but it's not like allocators are mmap'ing large fractions of available
memory. The trend is toward more and smaller mmap'd backing blocks to, e.g.,
improve address randomization.

> Another example is thread stacks. If you give each thread a one megabyte
> stack and make 100 threads, that's 100 megs of virtual space. But that one
> megabyte is a worst case that few, if any, of the threads will hit.

Fortunately GCC already supports segmented stacks. Such support would have
been there sooner were it not for overcommit reducing the apparent demand
for such a feature. Although, glibc doesn't seem to be too eager to jump
aboard. Perhaps they feel that a 5% performance hit is too high a cost to
get rid of applications randomly crashing under high loads.

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malloc jgk@panix.com (Joe keane) - 2012-02-16 21:04 +0000
  Re: malloc Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-02-16 13:21 -0800
    Re: malloc BGB <cr88192@hotmail.com> - 2012-02-16 16:16 -0700
      Re: malloc Paavo Helde <myfirstname@osa.pri.ee> - 2012-02-17 14:46 -0600
        Re: malloc Joshua Maurice <joshuamaurice@gmail.com> - 2012-02-17 14:51 -0800
          Re: malloc BGB <cr88192@hotmail.com> - 2012-02-17 19:58 -0700
            Re: malloc Paavo Helde <myfirstname@osa.pri.ee> - 2012-02-18 02:18 -0600
              Re: malloc BGB <cr88192@hotmail.com> - 2012-02-18 01:58 -0700
              Re: malloc glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-02-18 15:30 +0000
            Re: malloc Kaz Kylheku <kaz@kylheku.com> - 2012-02-18 10:36 +0000
              Re: malloc BGB <cr88192@hotmail.com> - 2012-02-18 10:53 -0700
            Re: malloc William Ahern <william@wilbur.25thandClement.com> - 2012-02-18 16:43 -0800
              Re: malloc BGB <cr88192@hotmail.com> - 2012-02-18 20:03 -0700
                Re: malloc Miles Bader <miles@gnu.org> - 2012-02-19 12:49 +0900
                Re: malloc William Ahern <william@wilbur.25thandClement.com> - 2012-02-18 21:08 -0800
                Re: malloc "io_x" <a@b.c.invalid> - 2012-02-19 18:23 +0100
                Re: malloc William Ahern <william@wilbur.25thandClement.com> - 2012-02-18 21:06 -0800
                Re: malloc BGB <cr88192@hotmail.com> - 2012-02-18 23:06 -0700
      Re: malloc "Asger Joergensen" <Junk@Asger-P.dk> - 2012-02-19 15:54 +0000
        Re: malloc BGB <cr88192@hotmail.com> - 2012-02-19 09:37 -0700
    Re: malloc Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-16 20:27 -0500
      Re: malloc Goran <goran.pusic@gmail.com> - 2012-02-16 23:40 -0800
        Re: malloc Keith Thompson <kst-u@mib.org> - 2012-02-17 02:26 -0800
          Re: malloc Goran <goran.pusic@gmail.com> - 2012-02-17 04:01 -0800
            Re: malloc Keith Thompson <kst-u@mib.org> - 2012-02-17 09:25 -0800
              Re: malloc Joshua Maurice <joshuamaurice@gmail.com> - 2012-02-17 14:49 -0800
                Re: malloc glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-02-18 01:43 +0000
                Re: malloc Kaz Kylheku <kaz@kylheku.com> - 2012-02-18 02:04 +0000
                Re: malloc scott@slp53.sl.home (Scott Lurndal) - 2012-02-18 03:04 +0000
                Re: malloc Robert Wessel <robertwessel2@yahoo.com> - 2012-02-17 22:15 -0600
                Re: malloc Joshua Maurice <joshuamaurice@gmail.com> - 2012-02-17 23:48 -0800
              Re: malloc Kaz Kylheku <kaz@kylheku.com> - 2012-02-17 23:29 +0000
                Re: malloc William Ahern <william@wilbur.25thandClement.com> - 2012-02-17 15:42 -0800
                Re: malloc "io_x" <a@b.c.invalid> - 2012-02-19 18:24 +0100
                Re: malloc Kleuske <kleuske@nowhere.net> - 2012-02-19 20:41 +0000
                Re: malloc James Kuyper <jameskuyper@verizon.net> - 2012-02-19 18:05 -0500
                Re: malloc "io_x" <a@b.c.invalid> - 2012-02-20 12:24 +0100
                Re: malloc Kleuske <kleuske@nowhere.net> - 2012-02-20 13:56 +0000
                Re: malloc "io_x" <a@b.c.invalid> - 2012-02-20 15:14 +0100
                Re: malloc William Ahern <william@wilbur.25thandClement.com> - 2012-02-19 15:19 -0800
        Re: malloc Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-17 08:06 -0500
          Re: malloc Goran <goran.pusic@gmail.com> - 2012-02-18 07:57 -0800
            Re: malloc Keith Thompson <kst-u@mib.org> - 2012-02-18 10:57 -0800
            Re: malloc Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-19 08:51 -0500
      Re: malloc Noob <root@127.0.0.1> - 2012-02-17 11:44 +0100
        Re: malloc "Joachim Schmitz" <nospam.jojo@schmitz-digital.de> - 2012-02-17 17:05 +0100
          Re: malloc pete <pfiland@mindspring.com> - 2012-02-17 11:19 -0500
            Re: malloc Noob <root@127.0.0.1> - 2012-02-17 17:30 +0100
          Re: malloc James Kuyper <jameskuyper@verizon.net> - 2012-02-17 11:31 -0500
      Re: malloc scott@slp53.sl.home (Scott Lurndal) - 2012-02-17 20:31 +0000
        Re: malloc Joshua Maurice <joshuamaurice@gmail.com> - 2012-02-18 00:01 -0800
          Re: malloc scott@slp53.sl.home (Scott Lurndal) - 2012-02-18 17:07 +0000
            Re: malloc Joshua Maurice <joshuamaurice@gmail.com> - 2012-02-18 18:14 -0800
              Re: malloc scott@slp53.sl.home (Scott Lurndal) - 2012-02-19 17:14 +0000
  Re: malloc Goran <goran.pusic@gmail.com> - 2012-02-16 23:43 -0800
  Re: malloc "io_x" <a@b.c.invalid> - 2012-02-19 16:44 +0100

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