Date: Fri, 10 Oct 2008 16:18:44 -0700
From: Gary Hade <garyhade@us.ibm.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Gary Hade <garyhade@us.ibm.com>, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org, y-goto@jp.fujitsu.com, pbadari@us.ibm.com,
       mel@csn.ul.ie, lcm@us.ibm.com, mingo@elte.hu, greg@kroah.com,
       dave@linux.vnet.ibm.com, nish.aravamudan@gmail.com
Subject: Re: [PATCH 1/2] [REPOST] mm: show node to memory section
	relationship with symlinks in sysfs
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On Fri, Oct 10, 2008 at 02:59:50PM -0700, Andrew Morton wrote:
> On Fri, 10 Oct 2008 14:33:57 -0700
> Gary Hade <garyhade@us.ibm.com> wrote:
> 
> > On Fri, Oct 10, 2008 at 12:42:39PM -0700, Andrew Morton wrote:
> > > On Thu, 9 Oct 2008 12:21:15 -0700
> > > Gary Hade <garyhade@us.ibm.com> wrote:
> > > 
> > > > Show node to memory section relationship with symlinks in sysfs
> > > > 
> > > > Add /sys/devices/system/node/nodeX/memoryY symlinks for all
> > > > the memory sections located on nodeX.  For example:
> > > > /sys/devices/system/node/node1/memory135 -> ../../memory/memory135
> > > > indicates that memory section 135 resides on node1.
> > > 
> > > I'm not seeing here a description of why the kernel needs this feature.
> > > Why is it useful?  How will it be used?  What value does it have to
> > > our users?
> > 
> > Sorry, I should have included that.  In our case, it is another
> > small step towards eventual total node removal.  We will need to
> > know which memory sections to offline for whatever node is targeted
> > for removal.  However, I suspect that exposing the node to section
> > information to user-level could be useful for other purposes.
> > For example, I have been thinking that using memory hotremove
> > functionality to modify the amount of available memory on specific
> > nodes without having to physically add/remove DIMMs might be useful
> > to those that test application or benchmark performance on a
> > multi-node system in various memory configurations.
> > 
> 
> hm, OK, thanks.  It does sound a bit thin, and if we merge this then
> not only do we get a porkier kernel,

Would you feel the same about the size increase if patch 2/2 (include
memory section subtree in sysfs with only sparsemem enabled) was
withdrawn?

Without patch 2/2 the size increase for non-Sparsemem or Sparsemem
wo/memory hotplug kernels is extremely small.  Even for memory hotplug
enabled kernels there is only a little extra code in ./drivers/base/node.o
which only gets linked into NUMA enabled kernels.  I can gather some numbers
if necessary.

> we also get a new userspace interface which we're then locked into.

True.

> 
> So I'm inclined to skip this change until we have a stronger need?

Of course, I'm not. :)

Gary

-- 
Gary Hade
System x Enablement
IBM Linux Technology Center
503-578-4503  IBM T/L: 775-4503
garyhade@us.ibm.com
http://www.ibm.com/linux/ltc

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