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    SubjectRe: Overcommitable memory??
    On Fri, 17 Mar 2000, Andreas Bombe wrote:

    > On Thu, Mar 16, 2000 at 11:04:23AM +0000, James Sutherland wrote:
    > > On 15 Mar 2000, Rask Ingemann Lambertsen wrote:
    > > > 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?
    > >
    > > Because it *IS* overcommitment of memory. You can have two processes, each
    > > with their 200Mb of data, in a machine with 256Mb RAM+swap, quite happily
    > > - until they start writing to it, at which point you discover you have
    > > overcommitted your memory, and things go wrong.
    >
    > He means avoiding overcommit by counting vm requirements but without
    > actually copying COW pages (denying a COW allocation if it could not
    > be faulted in at a later time). Resulting in vast areas of unused
    > RAM.

    Yes, I know. This does avoid the performance hit - but it still wastes
    obscene amounts of swap space unnecessarily, and makes the original
    problem worse by reducing the available amount of memory.

    On a WWW server with 100 Apache processes of 20Mb, for example, I would
    need 30Mb or so normally - or 2Gb with this strategy, even though 1.97Gb
    of this is never used. This means I will run out of memory a LOT sooner -
    I have 1.97Gb less VM than I otherwise would! This certainly doesn't help
    the original problem...


    James.


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