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    SubjectRe: Efficient IPC mechanism on Linux
    Arjan van de Ven wrote:
    > > The overhead implied by a memcpy() is the same, in the oder of magnitude,
    > > ***whatever*** kernel version you can develop.
    >
    > yes a copy of a page is about 3000 to 4000 cycles on an x86 box in the
    > uncached case. A pagetable operation (like the cpu setting the accessed
    > or dirty bit) is in that same order I suspect (maybe half this, but not
    > a lot less). Changing pagetable content is even more because all the
    > tlb's and internal cpu state will need to be flushed... which is also a
    > microcode operation for the cpu. And it's deadly in an SMP environment.

    I have just done a measurement on a 366MHz PII Celeron. The test is
    quite crude, but these numbers are upper bounds on the timings:

    mean 813 cycles to:

    page fault
    install zero page
    remove zero page

    (= read every page from MAP_ANON region and unmap, repeatedly)

    mean 11561 cycles to:

    page fault
    copy zero page
    install copy
    remove copy

    (= write every page from MAP_ANON region and unmap, repeatedly)

    I think that makes a clear case for avoiding page copies.

    -- Jamie
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