linux-kernel - Re: [RFC][PATCH 10/26] mm, mpol: Make mempolicy home-node aware

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Date:	Mon, 19 Mar 2012 10:16:41 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Dan Smith <danms@...ibm.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH 10/26] mm, mpol: Make mempolicy home-node aware

On Mon, 19 Mar 2012, Peter Zijlstra wrote:

> > A HOME_NODE policy would also help to ensure that existing applications
> > continue to work as expected. Given that people in the HPC industry and
> > elsewhere have been fine tuning around the scheduler for years this is a
> > desirable goal and ensures backward compatibility.
>
> I really have no idea what you're saying. Existing applications that use
> mbind/set_mempolicy already continue to function exactly like before,
> see how the new layer is below all that.

No they wont work the same way as before. Applications may be relying on
MPOL_DEFAULT behavior now expecting node local allocations. The home-node
functionality would cause a difference in behavior because it would
perform remote node allocs when a thread has been moved to a different
socket. The changes also cause migrations that may cause additional
latencies as well as change the location of memory in surprising ways for
the applications

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