Messages in this thread | | | Date | Tue, 16 Jun 2009 13:37:15 +0530 | From | Vaidyanathan Srinivasan <> | Subject | Re: [RFD PATCH 0/4] cpu: Bulk CPU Hotplug support. |
| |
* Andrew Morton <akpm@linux-foundation.org> [2009-06-15 23:23:18]:
> On Tue, 16 Jun 2009 11:08:39 +0530 Gautham R Shenoy <ego@in.ibm.com> wrote: > > > Currently on a ppc64 box with 16 CPUs, the time taken for > > a individual cpu-hotplug operation is as follows. > > > > # time echo 0 > /sys/devices/system/cpu/cpu2/online > > real 0m0.025s > > user 0m0.000s > > sys 0m0.002s > > > > # time echo 1 > /sys/devices/system/cpu/cpu2/online > > real 0m0.021s > > user 0m0.000s > > sys 0m0.000s > > Surprised. Do people really online and offline CPUs frequently enough > for this to be a problem?
Certainly not for hardware faults or hardware replacement, but cpu-hotplug interface is useful for changing system configuration to meet different objectives like
* Reduce system capacity to reduce average power and reduce heat
* Increasing number of cores and threads in a CPU package is leading to multiple cpu offline/online operations for any perceivable effect
* Dynamically change CPU configurations in virtualized environments
Ref:
[1] Saving power by cpu evacuation sched_max_capacity_pct=n http://lkml.org/lkml/2009/5/13/173
[2] Make offline cpus to go to deepest idle state using http://lkml.org/lkml/2009/5/22/431
[3] cpuset: add new API to change cpuset top group's cpus http://lkml.org/lkml/2009/5/19/54
For getting stuff off a certain CPU, cpu-hotplug framework seems to do the right thing. Identifying bottlenecks in the framework can significantly help other use cases.
--Vaidy
| |