From 5517d86bea237c1d7078840182d9ebc0fe4c1afc Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Tue, 8 May 2007 00:32:57 -0700 Subject: Speed up divides by cpu_power in scheduler I noticed expensive divides done in try_to_wakeup() and find_busiest_group() on a bi dual core Opteron machine (total of 4 cores), moderatly loaded (15.000 context switch per second) oprofile numbers : CPU: AMD64 processors, speed 2600.05 MHz (estimated) Counted CPU_CLK_UNHALTED events (Cycles outside of halt state) with a unit mask of 0x00 (No unit mask) count 50000 samples % symbol name ... 613914 1.0498 try_to_wake_up 834 0.0013 :ffffffff80227ae1: div %rcx 77513 0.1191 :ffffffff80227ae4: mov %rax,%r11 608893 1.0413 find_busiest_group 1841 0.0031 :ffffffff802260bf: div %rdi 140109 0.2394 :ffffffff802260c2: test %sil,%sil Some of these divides can use the reciprocal divides we introduced some time ago (currently used in slab AFAIK) We can assume a load will fit in a 32bits number, because with a SCHED_LOAD_SCALE=128 value, its still a theorical limit of 33554432 When/if we reach this limit one day, probably cpus will have a fast hardware divide and we can zap the reciprocal divide trick. Ingo suggested to rename cpu_power to __cpu_power to make clear it should not be modified without changing its reciprocal value too. I did not convert the divide in cpu_avg_load_per_task(), because tracking nr_running changes may be not worth it ? We could use a static table of 32 reciprocal values but it would add a conditional branch and table lookup. [akpm@linux-foundation.org: !SMP build fix] Signed-off-by: Eric Dumazet Acked-by: Ingo Molnar Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/sched.h | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) (limited to 'include') diff --git a/include/linux/sched.h b/include/linux/sched.h index 15ab3e03953..3d95c480f58 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -680,8 +680,14 @@ struct sched_group { /* * CPU power of this group, SCHED_LOAD_SCALE being max power for a * single CPU. This is read only (except for setup, hotplug CPU). + * Note : Never change cpu_power without recompute its reciprocal */ - unsigned long cpu_power; + unsigned int __cpu_power; + /* + * reciprocal value of cpu_power to avoid expensive divides + * (see include/linux/reciprocal_div.h) + */ + u32 reciprocal_cpu_power; }; struct sched_domain { -- cgit v1.2.3-70-g09d2