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    SubjectRe: [RFC] syscalls, x86: Add __NR_kcmp syscall
    On Wed, Jan 18, 2012 at 02:05:31PM -0800, david@lang.hm wrote:
    ...
    > >>
    > >>Why on Earth would user space need to know which order in memory certain
    > >>kernel objects are?
    > >
    > >For checkpoint restart and for some other kinds of introspection what is
    > >needed is a comparison function to see if two processes share the same
    > >object. The most interesting of these objects from a checkpoint restart case
    > >are file descriptors, and there can be a lot of file descriptors.
    > >
    > >The order in memory does not matter. What does matter is that the
    > >comparison function return some ordering between objects. The algorithm
    > >for figuring out of N items which of them are duplicates is O(N^2) if
    > >the comparison function can only return equal or not equal. The
    > >algorithm for finding duplications is only O(NlogN) if the comparison
    > >function will return an ordering among the objects.
    >
    > so what you really want is a syscall that can take a list of objects
    > instead of having to do a syscall per object. right?
    >

    It doesn't matter. Even if we take a list of objects the kernel either
    should return us some ordering info or find duplicates, in any case it
    makes things more complex i think. So we wanted to bring some minimum
    into kernel leaving the rest of work to user-space.

    (and, btw, Eric, I'm really sorry that I didn't mentioned this sorting
    problem at the very first email).

    Cyrill


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