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Re: malloc

Cross-posted to 2 groups.

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On 18 Feb 2012 03:04:14 GMT, scott@slp53.sl.home (Scott Lurndal)
wrote:

>Joshua Maurice <joshuamaurice@gmail.com> writes:
>>On Feb 17, 9:25=A0am, Keith Thompson <ks...@mib.org> wrote:
>>> If you call malloc() and it overcommits, it won't crash the
>>> program until you access the allocated memory. =A0(The rationale for
>>> overcommitting is that most programs don't actually use most of
>>> the memory the memory the allocate. =A0I find that odd)
>>
>>Really? The explanation that I'm most familiar with is that most fork
>>calls are immediately followed by exec, and thus if you're low on
>>memory, then a large process cannot spawn a new process without
>>overcommit because the only process create primitive is fork, which
>>"copies" the memory space of the parent process.
>
>You're confusing overcommit with copy-on-write.  Fork uses COW[*] in
>which the parent and child share the physical pages until the child
>writes to one - at that point, they child gets a copy (and an allocation
>occurs which may fail at that point if memory and swap are exhausted).
>
>Overcommit was allowed to support sparse arrays which are common
>with some workloads.


I don't think there's much daylight between the two.  In both cases a
process has been implicitly allocated virtual pages, without
(necessarily) the presence of enough actual pages (real and/or disk)
to make good on that allocation, and the system can fail in the future
when the application does something apparently innocuous, like try to
write to a page it appears to have been allocated.

And while I'm not familiar with the implementation of all over-commit
schemes, at least a couple I've seen actually map all allocated pages
to a read-only pages full of zeros, and then use the COW mechanism to
allocate actual pages when they're written to.  This has the advantage
of not requiring a second mechanism, and possibly further delaying the
actual allocation (as an ordinary read does not trigger it).

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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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