Commit db1b1fef authored Mar 31, 2006 by Jack Steiner Committed by Linus Torvalds Mar 31, 2006

[PATCH] sched: reduce overhead of calc_load



Currently, count_active_tasks() calls both nr_running() &
nr_interruptible().  Each of these functions does a "for_each_cpu" & reads
values from the runqueue of each cpu.  Although this is not a lot of
instructions, each runqueue may be located on different node.  Depending on
the architecture, a unique TLB entry may be required to access each
runqueue.

Since there may be more runqueues than cpu TLB entries, a scan of all
runqueues can trash the TLB.  Each memory reference incurs a TLB miss &
refill.

In addition, the runqueue cacheline that contains nr_running &
nr_uninterruptible may be evicted from the cache between the two passes.
This causes unnecessary cache misses.

Combining nr_running() & nr_interruptible() into a single function
substantially reduces the TLB & cache misses on large systems.  This should
have no measureable effect on smaller systems.

On a 128p IA64 system running a memory stress workload, the new function
reduced the overhead of calc_load() from 605 usec/call to 324 usec/call.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent 3055adda

include/linux/sched.h

+1 −0

Original line number	Diff line number	Diff line
		@@ -100,6 +100,7 @@ DECLARE_PER_CPU(unsigned long, process_counts);
		extern int nr_processes(void);
		extern unsigned long nr_running(void);
		extern unsigned long nr_uninterruptible(void);
		extern unsigned long nr_active(void);
		extern unsigned long nr_iowait(void);

		#include <linux/time.h>

kernel/sched.c

+15 −0

Original line number	Diff line number	Diff line
		@@ -1658,6 +1658,21 @@ unsigned long nr_iowait(void)
		return sum;
		}

		unsigned long nr_active(void)
		{
		unsigned long i, running = 0, uninterruptible = 0;

		for_each_online_cpu(i) {
		running += cpu_rq(i)->nr_running;
		uninterruptible += cpu_rq(i)->nr_uninterruptible;
		}

		if (unlikely((long)uninterruptible < 0))
		uninterruptible = 0;

		return running + uninterruptible;
		}

		#ifdef CONFIG_SMP

		/*

kernel/timer.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -825,7 +825,7 @@ void update_process_times(int user_tick)
		*/
		static unsigned long count_active_tasks(void)
		{
		return (nr_running() + nr_uninterruptible()) * FIXED_1;
		return nr_active() * FIXED_1;
		}

		/*

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