Commit cd7175ed authored Dec 13, 2006 by Eric Dumazet Committed by Linus Torvalds Dec 13, 2006

[PATCH] Optimize calc_load()



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>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: "Siddha, Suresh B" <suresh.b.siddha@intel.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent f988443a

kernel/timer.c

+9 −5

Original line number	Diff line number	Diff line
		@@ -1146,11 +1146,15 @@ static inline void calc_load(unsigned long ticks)
		unsigned long active_tasks; /* fixed-point */
		static int count = LOAD_FREQ;

		count -= ticks;
		if (unlikely(count < 0)) {
		active_tasks = count_active_tasks();
		for (count -= ticks; count < 0; count += LOAD_FREQ) {
		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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