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    SubjectRe: [PATCH] ftrace: print sample std dev when function profiling
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    On Tue, 2010-04-27 at 17:06 -0400, Chase Douglas wrote:
    > On Tue, Apr 27, 2010 at 4:50 PM, Steven Rostedt <rostedt@goodmis.org> wrote:
    > > On Mon, 2010-04-26 at 14:02 -0400, Chase Douglas wrote:
    > >
    > >> + /* Sample standard deviation (s^2) */
    > >> + if (rec->counter <= 1)
    > >> + stddev = 0;
    > >> + else {
    > >> + stddev = rec->time_squared - rec->counter * avg * avg;
    > >> + do_div(stddev, (rec->counter - 1) * 1000); /* ns^2 -> us^2 */
    > >
    > > Shouldn't this be:
    > >
    > > do_div(stddev, (rec->counter - 1) * 1000000); ?
    > >
    > > (x / 1000)^2 == x^2 / 1000^2
    >
    > The trace_print_graph_duration function divides the value by 1000
    > again to display in us units. I could make the comment more clear, but
    > I didn't want to make a big fuss over a unit conversion.
    >
    > I also figured this out *after* I had some really wrong looking
    > numbers :). I made sure this patch produced the correct value after
    > two events, so I assume the math is correct.

    OK, I see that now. I'll add this patch but I'll also fix the comment.

    Thanks,

    -- Steve




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