[RFC PATCH 0/1]: timers: Timer Migration
From: Arun R Bharadwaj
Date: Tue Sep 16 2008 - 05:14:25 EST
Hi,
An idle cpu on which device drivers have initialized timers, has to be frequently woken up to service the timers. So, consolidation of timers onto a fewer number of cpus is important. Migration of
timers from idle cpus onto lesser idle cpus is necessary. Currently, timers are migrated during the cpu offline operation. However cpu-hotplug for the sake of idle system power management is too
heavy. So, this patch implements a lightweight timer migration framework.
Also, in a multi-core, multi-package system, it is always desirable to have all the timers firing on the cpus present in the same package. This would enable us to place the idle package in deep sleep
state. So, migration of timers is required for this.
A per_cpu sysfs hook is created at /sys/devices/system/cpu/cpuX/timer_migration. By echo-ing 1 to cpuX/timer_migration, all regular and hrtimers from cpuX are migrated to cpu0. The fact that all
timers are migrated from cpuX to cpu0 is demonstrated by making a note of /proc/interrupts.
$:echo 1 > /sys/devices/system/cpu/cpu3/timer_migration
$:cat /proc/interrupts > file1; sleep 30 ; cat /proc/interrupts > file2
$:diff file1 file2 | grep LOC
< LOC: 5276 2100 2676 1647 2415 1803 2275 2977 Local timer interrupts
> LOC: 6991 2195 2774 1647 2562 1900 2497 3075 Local timer interrupts
As we can observe, first we are enabling timer migration in cpu3. As a result, the number of local timer interrupts on cpu3 over the next 30 seconds are zero. cpu3 has been totally rid of all regular
and hrtimers.
Please refer to the following paper for details:
http://ols.fedoraproject.org/OLS/Reprints-2008/srinivasan1-reprint.pdf
thanks,
Arun
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