Date: Thu, 9 Jun 2011 11:52:59 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Ankita Garg <ankita@in.ibm.com>, linux-arm-kernel@lists.infradead.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-pm@lists.linux-foundation.org, svaidy@linux.vnet.ibm.com,
        thomas.abraham@linaro.org
Subject: Re: [PATCH 00/10] mm: Linux VM Infrastructure to support Memory
 Power Management
Message-ID: <20110609185259.GA29287@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.com
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 <20110528005640.9076c0b1.akpm@linux-foundation.org>
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On Sat, May 28, 2011 at 12:56:40AM -0700, Andrew Morton wrote:
> On Fri, 27 May 2011 18:01:28 +0530 Ankita Garg <ankita@in.ibm.com> wrote:
> 
> > This patchset proposes a generic memory regions infrastructure that can be
> > used to tag boundaries of memory blocks which belongs to a specific memory
> > power management domain and further enable exploitation of platform memory
> > power management capabilities.
> 
> A couple of quick thoughts...
> 
> I'm seeing no estimate of how much energy we might save when this work
> is completed.  But saving energy is the entire point of the entire
> patchset!  So please spend some time thinking about that and update and
> maintain the [patch 0/n] description so others can get some idea of the
> benefit we might get from all of this.  That estimate should include an
> estimate of what proportion of machines are likely to have hardware
> which can use this feature and in what timeframe.
> 
> IOW, if it saves one microwatt on 0.001% of machines, not interested ;)

FWIW, I have seen estimates on the order of a 5% reduction in power
consumption for some common types of embedded devices.

							Thanx, Paul

> Also, all this code appears to be enabled on all machines?  So machines
> which don't have the requisite hardware still carry any additional
> overhead which is added here.  I can see that ifdeffing a feature like
> this would be ghastly but please also have a think about the
> implications of this and add that discussion also.  
> 
> If possible, it would be good to think up some microbenchmarks which
> probe the worst-case performance impact and describe those and present
> the results.  So others can gain an understanding of the runtime costs.
> 
> 
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