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    SubjectRe: [PATCH][RFC] vsprintf: unify the format decoding layer for its 3 users
    On Fri, 27 Feb 2009 07:19:37 +0100 Frederic Weisbecker <fweisbec@gmail.com> wrote:

    >
    > An new optimization is making its way to ftrace. Its purpose is to
    > make ftrace_printk() consuming less memory and be faster.
    >
    > Written by Lai Jiangshan, the approach is to delay the formatting
    > job from tracing time to output time.
    > Currently, a call to ftrace_printk will format the whole string and
    > insert it into the ring buffer.

    It does that? eek.

    > Then you can read it on /debug/tracing/trace
    > file.
    >
    > The new implementation stores the address of the format string and
    > the binary parameters into the ring buffer, making the packet more compact
    > and faster to insert.
    > Later, when the user exports the traces, the format string is retrieved
    > with the binary parameters and the formatting job is eventually done.
    >
    > Here is the result of a small comparative benchmark while putting the following
    > ftrace_printk on the timer interrupt. ftrace_printk is the old implementation,
    > ftrace_bprintk is a the new one:
    >
    > ftrace_printk("This is the timer interrupt: %llu", jiffies_64);
    >
    > After some time running on low load (no X, no really active processes):
    >
    > ftrace_printk: duration average: 2044 ns, avg of bytes stored per entry: 39
    > ftrace_bprintk: duration average: 1426 ns, avg of bytes stored per entry: 16
    >
    > Higher load (started X and launched a cat running on an X console looping on
    > traces printing):
    >
    > ftrace_printk: duration average: 8812 ns
    > ftrace_bprintk: duration average: 2611 ns
    >
    > Which means the new implementation can be 70 % faster on higher load.
    > And it consumes lesser memory on the ring buffer.
    >
    > The curent implementation rewrites a lot of format decoding bits from
    > vsnprintf() function, making now 3 differents functions to maintain
    > in their duplicated parts of printf format decoding bits.

    <looks for ftrace_bprintk() in linux-next, fails>

    Why does the current ftrace_bprintk() need to hack around in (or
    duplicate) vprintk() internals? It's a bit grubby, but by placing an
    upper bound on the number of args, it could simply call vscnprintf()
    directly?



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