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    SubjectRe: Overcommitable memory??
    Matija Nalis wrote:
    > On 17 Mar 2000 00:27:42 +0100, Rask Ingemann Lambertsen <> wrote:
    > >Den 15-Mar-00 11:49:45 skrev James Sutherland følgende om "Re: Overcommitable memory??":
    > >> On Wed, 15 Mar 2000, David Whysong wrote:
    > >
    > >>> The only possibilities are to (a) enforce hard memory limits with no
    > >>> overcommit, thereby wasting large amounts of swap space that will never
    > >>> get used while not really solving the problem,
    > >
    > >> As well as *destroying* performance (you'd effectively eliminate COW
    > >> capability on fork - if your 500Mb simulation wants to fork a 100k mailer
    > >> process to send you an update, the kernel has to allocate and copy 500Mb
    > >> of RAM/swap first, then discard it all again.)
    > >
    > > Not at all. COW is a performance optimisation which does not depend on
    > >overcommitment of memory in any way. Why would you want to turn it off?
    > Say you have 800MB virtual RAM, and you simulation currently uses 500MB.
    > And now, it tries to fork(2). Do you allow it, or does fork fail with
    > -ENOMEM ?
    Had exactly this problem in SoftWindows on IRIX. It would
    fork to run some trivial external application, or a sound-process.
    It would die running out of memory (SWin mem image was often large).
    Solution was to convert to using
    'sproc', where amount of process sharing can be user defined -- something
    like a heavy-weight thread. If all you were going to do was to exec
    another proc, you could set the new proc to "share all mem":

    PR_SADDR All virtual space attributes (shared memory, mapped files, data
    space) are shared. If one process in a share group attaches to
    a shared memory segment, all processes in the group can access
    that segment.
    Then do the exec call. Thus no overcommit.
    Linda A Walsh | Trust Technology, Core Linux, SGI | Voice: (650) 933-5338

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