linux-kernel - Re: [RFD PATCH 0/4] cpu: Bulk CPU Hotplug support.

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Date:	Tue, 16 Jun 2009 14:00:59 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Balbir Singh <balbir@...ibm.com>,
	Rusty Russel <rusty@...tcorp.com.au>,
	Nathan Lynch <ntl@...ox.com>, Ingo Molnar <mingo@...e.hu>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Shoahua Li <shaohua.li@...ux.com>
Subject: Re: [RFD PATCH 0/4] cpu: Bulk CPU Hotplug support.

On Tue, Jun 16, 2009 at 01:37:15PM +0530, Vaidyanathan Srinivasan wrote:
> * Andrew Morton <akpm@...ux-foundation.org> [2009-06-15 23:23:18]:
> 
> > On Tue, 16 Jun 2009 11:08:39 +0530 Gautham R Shenoy <ego@...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

Perhaps also reducing boot-up time?  If I am correctly interpreting the
above numbers, an eight-CPU system would be consuming 175 milliseconds
bringing up the seven non-boot CPUs.  Reducing this by 150 milliseconds
might be of interest to some people.  ;-)

							Thanx, Paul

> 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
> 
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