Date: Fri, 10 Oct 2008 16:32:30 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Gary Hade <garyhade@us.ibm.com>
Cc: 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, 10 Oct 2008 16:18:44 -0700
Gary Hade <garyhade@us.ibm.com> wrote:

> 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 withdrawn?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.

Size is probably a minor issue on memory-hotpluggable machines.

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

That's a bigger issue.  The later we leave this sort of thing, the more
information we have.

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