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Date: Wed, 18 Mar 2009 13:35:39 +0530
From: Bharata B Rao <bharata@linux.vnet.ibm.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, balbir@linux.vnet.ibm.com,
       Li Zefan <lizf@cn.fujitsu.com>, linux-kernel@vger.kernel.org,
       Dhaval Giani <dhaval@linux.vnet.ibm.com>,
       Paul Menage <menage@google.com>, Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in
	cpuacct_charge() safe when rcupreempt is used.
Message-ID: <20090318080539.GD3960@in.ibm.com>
Reply-To: bharata@linux.vnet.ibm.com
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On Wed, Mar 18, 2009 at 04:08:27PM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 18 Mar 2009 10:18:01 +0530
> Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:
> > Looks ok to me, but will wait for experts' comments.
> > 
> > However, I did a quick measurement of read times with percpu_counter_read()
> > (no readside lock) and percpu_counter_sum() (readside lock) and I don't
> > see a major slowdown with percpu_counter_sum().
> > 
> > Time taken for 100 reads of cpuacct.stat with 1s delay b/n every read.
> > percpu_counter_read() - 9845 us
> > percpu_counter_sum() - 9974 us
> > 
> Then, almost 1 us overhead per read().....Hmm, seems big (as counter).

Well some cost for correct and accurate counter :)

BTW, I did a few more iterations and I don't see consistent numbers.
The results from 7 runs look like this:

percpu_counter_read() - 11325, 11549, 5939, 9999, 7129, 7758, 11385 us
percpu_counter_sum() - 8655, 9201, 8705, 11766, 10619, 9186, 8890 us 

Regards,
Bharata.