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    SubjectRe: broken VM in 2.4.10-pre9
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    Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

    > > On September 17, 2001 06:03 pm, Eric W. Biederman wrote:
    > > > In linux we have avoided reverse maps (unlike the BSD's) which tends
    > > > to make the common case fast at the expense of making it more
    > > > difficult to handle times when the VM system is under extreme load and
    > > > we are swapping etc.
    > >
    > > What do you suppose is the cost of the reverse map? I get the impression you
    >
    > > think it's more expensive than it is.
    >
    > We can keep the typical page table cost lower than now (including reverse
    > maps) just by doing some common sense small cleanups to get the page struct
    > down to 48 bytes on x86

    While there is a size cost I suspect you will notice reverse maps
    a lot more in operations like fork where having them tripples the amount
    of memory that you need to copy. So you should see a double or more
    in the time it takes to do a fork.

    That I think is a significant cost.

    Eric

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