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    SubjectRe: [PATCH v2] proc: speedup /proc/stat handling
    On Wed, 25 Jan 2012 01:01:23 +0100
    Eric Dumazet <eric.dumazet@gmail.com> wrote:

    > On a typical 16 cpus machine, "cat /proc/stat" gives more than 4096
    > bytes, and is slow :
    >
    > # strace -T -o /tmp/STRACE cat /proc/stat | wc -c
    > 5826
    > # grep "cpu " /tmp/STRACE
    > read(0, "cpu 1949310 19 2144714 12117253"..., 32768) = 5826 <0.001504>
    >
    >
    > Thats partly because show_stat() must be called twice since initial
    > buffer size is too small (4096 bytes for less than 32 possible cpus)
    >
    > Fix this by :
    >
    > 1) Taking into account nr_irqs in the initial buffer sizing.
    >
    > 2) Using ksize() to allow better filling of initial buffer.
    >
    >
    > Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
    > Cc: Glauber Costa <glommer@parallels.com>
    > Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>
    > Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Paul Tuner <pjt@google.com>
    > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
    > Cc: Ingo Molnar <mingo@elte.hu>


    Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

    BTW, what is the reason of this change ?

    > - unsigned size = 4096 * (1 + num_possible_cpus() / 32);
    > + unsigned size = 1024 + 128 * num_possible_cpus();

    I think size of buffer is affected by the number of online cpus.
    (Maybe 128 is enough but please add comment why 128 ?)

    And
    > size += 2 * nr_irqs;

    If this assumption fails, the issue comes again...


    -Kame



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