Messages in this thread | | | Date | Thu, 25 Oct 2007 17:28:02 -0700 (PDT) | From | Christoph Lameter <> | Subject | Re: [patch 2/2] cpusets: add interleave_over_allowed option |
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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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