Messages in this thread Patch in this message | | | Date | Fri, 17 Mar 2006 09:26:11 -0600 | From | Jack Steiner <> | Subject | Re: [PATCH] - 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.
(Same as previous patch except reorder extern as requested by Ingo)
Signed-off-by: Jack Steiner <steiner@sgi.com> Acked-by: Ingo Molnar <mingo@elte.hu>
include/linux/sched.h | 1 + kernel/sched.c | 16 ++++++++++++++++ kernel/timer.c | 8 +++++++- 3 files changed, 24 insertions(+), 1 deletion(-)
Index: linux/include/linux/sched.h =================================================================== --- linux.orig/include/linux/sched.h 2006-03-16 16:17:55.982340489 -0600 +++ linux/include/linux/sched.h 2006-03-17 09:23:44.847612520 -0600 @@ -99,6 +99,7 @@ DECLARE_PER_CPU(unsigned long, process_c 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> Index: linux/kernel/sched.c =================================================================== --- linux.orig/kernel/sched.c 2006-03-16 16:17:56.376832371 -0600 +++ linux/kernel/sched.c 2006-03-16 16:26:01.141002841 -0600 @@ -1655,6 +1655,22 @@ 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 /* Index: linux/kernel/timer.c =================================================================== --- linux.orig/kernel/timer.c 2006-03-16 16:17:56.385620556 -0600 +++ linux/kernel/timer.c 2006-03-16 16:26:01.141979306 -0600 @@ -849,7 +849,7 @@ 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; } /* - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |