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    SubjectRe: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods


    On Mon, 15 Jun 2009, Ingo Molnar wrote:
    >
    > See the numbers in the other mail: about 33 million pagefaults
    > happen in a typical kernel build - that's ~400K/sec - and that is
    > not a particularly really pagefault-heavy workload.

    Did you do any function-level profiles?

    Last I looked at it, the real cost of page faults were all in the memory
    copies and page clearing, and while it would be nice to speed up the
    kernel entry and exit, the few tens of cycles we might be able to get from
    there really aren't all that important.

    38 million page faults may sound like a lot, but put it in other terms: if
    we get rid of 20 cycles for each page fault, that's still not a lot of
    actual time. Lookie here at your own numbers:

    38465628 page-faults # 0.027 M/sec
    4374762924204 cycles # 3029.025 M/sec

    Now, if we shave 20 cycles off each page fault, that is still just, what,
    0.018% or something? Not really that impressive in the end.

    So I'm all for optimizing the kernel entry/exit paths, but ..

    Linus


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