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    SubjectRe: Linux Kernel Markers - performance characterization with large IO load on large-ish system
    Mathieu Desnoyers wrote:
    >> remember that we have seen and discussed something like this before,
    >> it's still a puzzle to me...
    >>
    >>
    > I do wonder about that performance _increase_ with blktrace enabled. I
    >
    > Interesting question indeed.
    >
    > In those tests, when blktrace is running, are the relay buffers only
    > written to or they are also read ?
    >

    blktrace (the utility) was running too - so the relay buffere /were/
    being read and stored out to disk elsewhere.

    > Running the tests without consuming the buffers (in overwrite mode)
    > would tell us more about the nature of the disturbance causing the
    > performance increase.
    >

    I'd have to write a utility to enable the traces, but then not read. Let
    me think about that.

    > Also, a kernel trace could help us understand more thoroughly what is
    > happening there.. is it caused by the scheduler ? memory allocation ?
    > data cache alignment ?
    >

    Yep - when I get some time, I'll look into that. [Clearly not a gating
    issue for marker support...]

    > I would suggest that you try aligning the block layer data structures
    > accessed by blktrace on L2 cacheline size and compare the results (when
    > blktrace is disabled).
    >

    Again, when I get some time! :-)

    Alan
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