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    SubjectRe: Note describing poor dcache utilization under high memory pressure
    On January 28, 2002 10:43 pm, Rik van Riel wrote:
    > On Mon, 28 Jan 2002, Rick Stevens wrote:
    > > Daniel Phillips wrote:
    > > [snip]
    > [page table COW description]
    > > Perhaps I'm missing this, but I read that as the child gets a reference
    > > to the parent's memory. If the child attempts a write, then new memory
    > > is allocated, data copied and the write occurs to this new memory. As
    > > I read this, it's only invoked on a child write.
    > >
    > > Would this not leave a hole where the parent could write and, since the
    > > child shares that memory, the new data would be read by the child? Sort
    > > of a hidden shm segment? If so, I think we've got problems brewing.
    > > Now, if a parent write causes the same behaviour as a child write, then
    > > my point is moot.
    > Daniel and I discussed this issue when Daniel first came up with
    > the idea of doing page table COW. He seemed a bit confused by
    > fork semantics when we first discussed this idea, too ;)

    Oh yes, I admit it, confused is me. That way I avoid heading off in
    directions that people have already gone, and found nothing ;-)

    > You're right though, both parent and child need to react in the
    > same way, preferably _without_ having to walk all of the parent's
    > page tables and mark them read-only ...

    Yes, and look at the algorithm as I've stated it, it's symmetric with respect
    to parent and child. Getting it into this simple and robust form took a lot
    of work, and as you know, my initial attempts were complex and, yes, confused.

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