linux-kernel - Re: [patch 3/4] mempolicy: add MPOL_F_STATIC

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Date:	Thu, 14 Feb 2008 04:26:43 -0600
From:	Paul Jackson <pj@....com>
To:	Paul Jackson <pj@....com>
Cc:	rientjes@...gle.com, Lee.Schermerhorn@...com,
	akpm@...ux-foundation.org, clameter@....com, ak@...e.de,
	linux-kernel@...r.kernel.org, mel@....ul.ie
Subject: Re: [patch 3/4] mempolicy: add MPOL_F_STATIC_NODES flag

Paul, responding (incorrectly) to David:
> > So, for example, if the task is bound by mems 1-3, and it asks for 
> > MPOL_INTERLEAVE over 2-4, then initially the mempolicy is only effected 
> > over node 3 and if it's later expanded to mems 1-8, then the mempolicy is 
> > effected over nodes 3-5, right?
> > 
> > And if the mems change to 3-8, the mempolicy is remapped to 5-7 even 
> > though 3-5 (which it already was interleaving over) is still accessible?
> 
> Yes and yes, for this cpuset relative numbering mode.

Paul, correcting himself ...

No and yes.  The manner in which too many nodes (as requested in a
RELATIVE mask) are folded into too small a cpuset is not actually
that critical, so long as it doesn't come up empty.  However, what
I'll be recommending, in a follow-up patch, will be folding the
larger set into the smaller one modulo the size of the smaller one.

In your example above, the requested policy asked for nodes 2-4,
so that means it is trying to lay out a memory policy with at least
five (0-4) nodes in consideration.  But the cpuset is only allowing
three (1-3) nodes at the moment.  So for each requested node, we take
its number modulo three to see which of the available nodes to include
in the interleave.

  Given
	N: how many nodes are allowed by the tasks cpuset (3, here)

	   n		m := (n % N)	r := m-th set node in allowed
	requested	  mod 3		result (in set 1, 2, 3 allowed)
	   2		    2	  		3
	   3		    0			1
	   4		    1			2

Hence your first example would result in an interleave over physical
nodes 1, 2, and 3 (the last column above.) ... not just over 3.

I intend to post patches you can use to lib/bitmap.c and the related
cpumask and nodemask apparatus that compute the above for you.

-- 
                  I won't rest till it's the best ...
                  Programmer, Linux Scalability
                  Paul Jackson <pj@....com> 1.940.382.4214
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