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    Subject[PATCH] Optimize calc_load()
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    calc_load() is called by timer interrupt to update avenrun[].
    It currently calls nr_active() at each timer tick (HZ per second), while the
    update of avenrun[] is done only once every 5 seconds. (LOAD_FREQ=5 Hz)

    nr_active() is quite expensive on SMP machines, since it has to sum up
    nr_running and nr_uninterruptible of all online CPUS, bringing foreign dirty
    cache lines.

    This patch is an optimization of calc_load() so that nr_active() is called
    only if we need it.

    The use of unlikely() is welcome since the condition is true only once every
    5*HZ time.

    Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
    --- linux-2.6.19/kernel/timer.c 2006-12-11 12:27:28.000000000 +0100
    +++ linux-2.6.19-ed/kernel/timer.c 2006-12-11 12:29:11.000000000 +0100
    @@ -1008,11 +1008,15 @@ static inline void calc_load(unsigned lo
    unsigned long active_tasks; /* fixed-point */
    static int count = LOAD_FREQ;

    - active_tasks = count_active_tasks();
    - for (count -= ticks; count < 0; count += LOAD_FREQ) {
    - CALC_LOAD(avenrun[0], EXP_1, active_tasks);
    - CALC_LOAD(avenrun[1], EXP_5, active_tasks);
    - CALC_LOAD(avenrun[2], EXP_15, active_tasks);
    + count -= ticks;
    + if (unlikely(count < 0)) {
    + active_tasks = count_active_tasks();
    + do {
    + CALC_LOAD(avenrun[0], EXP_1, active_tasks);
    + CALC_LOAD(avenrun[1], EXP_5, active_tasks);
    + CALC_LOAD(avenrun[2], EXP_15, active_tasks);
    + count += LOAD_FREQ;
    + } while (count < 0);
    }
    }
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