Re: [patch 2/2] cpusets: add interleave_over_allowed option

[Christoph Lameter <clameter@sgi.com>]

On Thu, 25 Oct 2007, David Rientjes wrote:

> The problem occurs when you add cpusets into the mix and permit the 
> allowed nodes to change without knowledge to the application.  Right now, 
> a simple remap is done so if the cardinality of the set of nodes 
> decreases, you're interleaving over a smaller number of nodes.  If the 
> cardinality increases, your interleaved nodemask isn't expanded.  That's 
> the problem that we're facing.  The remap itself is troublesome because it 
> doesn't take into account the user's desire for a custom nodemask to be 
> used anyway; it could remap an interleaved policy over several nodes that 
> will already be contended with one another.

Right. So I think we are fine if the application cannot setup boundaries 
for interleave.


> Normally, MPOL_INTERLEAVE is used to reduce bus contention to improve the 
> throughput of the application.  If you remap the number of nodes to 
> interleave over, which is currently how it's done when mems_allowed 
> changes, you could actually be increasing latency because you're 
> interleaving over the same bus.

Well you may hit some nodes more than others so a slight performance 
degradataion.

> This isn't a memory policy problem because all it does is effect a 
> specific policy over a set of nodes.  With my change, cpusets are required 
> to update the interleaved nodemask if the user specified that they desire 
> the feature with interleave_over_allowed.  Cpusets are, after all, the 
> ones that changed the mems_allowed in the first place and invalidated our 
> custom interleave policy.  We simply can't make inferences about what we 
> should do, so we allow the creator of the cpuset to specify it for us.  So 
> the proper place to modify an interleaved policy is in cpusets and not 
> mempolicy itself.

With that MPOL_INTERLEAVE would be context dependent and no longer 
needs translation. Lee had similar ideas. Lee: Could we make 
MPOL_INTERLEAVE generally cpuset context dependent?

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