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

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Date:	Wed, 24 Jun 2009 17:02:40 +0200
From:	Pavel Machek <pavel@....cz>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	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 2009-06-16 14:00:59, Paul E. McKenney wrote:
> 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.  ;-)

...also it should save 300msec from s2ram cycle. Actually maybe
suspend code should be modified first,  as it can demonstrate the
changes without kernel<-> user interface changing?

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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