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    SubjectRe: Intel P6 vs P7 system call performance
    dean gaudet wrote:
    > On Mon, 16 Dec 2002, Linus Torvalds wrote:
    >
    >>It's not as good as a pure user-mode solution using tsc could be, but
    >>we've seen the kinds of complexities that has with multi-CPU systems, and
    >>they are so painful that I suspect the sysenter approach is a lot more
    >>palatable even if it doesn't allow for the absolute best theoretical
    >>numbers.
    >
    > don't many of the multi-CPU problems with tsc go away because you've got a
    > per-cpu physical page for the vsyscall?
    >
    > i.e. per-cpu tsc epoch and scaling can be set on that page.
    >
    > the only trouble i know of is what happens when an interrupt occurs and
    > the task is rescheduled on another cpu... in theory you could test %eip
    > against 0xfffffxxx and "rollback" (or complete) any incomplete
    > gettimeofday call prior to saving a task's state. but i bet that test is
    > undesirable on all interrupt paths.
    >

    Exactly. This is a real problem.

    -hpa

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