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SubjectRe: [patch 2/2] cpusets: add interleave_over_allowed option
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On Monday 29 October 2007 20:35:58 Paul Jackson wrote:
> Lee wrote:
> > 2. As this thread progresses, you've discussed relaxing the requirement
> > that applications pass a valid subset of mems_allowed. I.e., something
> > that was illegal becomes legal. An API change, I think. But, a
> > backward compatible one, so that's OK, right? :-)
>
> The more I have stared at this, the more certain I've become that we
> need to make the mbind/mempolicy calls modal -- the default mode
> continues to interpret node numbers and masks just as these calls do
> now, and the alternative mode provides the so called "Choice B",
> which takes node numbers and masks as if the task owned the entire
> system, and then the kernel internally and automatically scrunches
> those masks down to whatever happens to be the current cpuset of
> the task.

So the user space asks for 8 nodes because it knows the machine
has that many from /sys and it only gets 4 if a cpuset says so? That's
just bad semantics. And is not likely to make the user programs happy.

I don't think you'll get around to teaching user space (or rather libnuma)
about cpusets and let it handle it.

From the libnuma perspective the machine size would be essentially
current cpuset size.

On the syscall level I don't think it makes much sense to change though.

The alternative would be to throw out the complete cpuset concept and go for
virtual nodes inside containers with virtualized /sys.

-Andi


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