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

Paul Jackson
2007-10-27T21:23:10Z

David wrote:
> I prefer Choice B because it does not force mempolicies to have any
> dependence on cpusets with regard to what nodemask is passed.

Yes, well said.

> It would be very good to store the passed nodemask to set_mempolicy in
> struct mempolicy,

Yes - that's what I'm intending to do.

> If the cpuset has fewer than four nodes, the behavior
> should be undefined (probably implemented to just cycle the set of
> mems_allowed until you reach the fourth entry).

I do intend to implement it as you suggest. See the lib/bitmap.c
routines bitmap_remap() and bitmap_bitremap(), and the nodemask
wrappers for these, nodes_remap() and node_remap(). They will
define the cycling, or I sometimes call it folding.

I would have tended to make this folding a defined part of the API,
though I will grant that the possibility of being lazy and forgetting
to document it seems attractive (less to document ;).

> That [running in a cpuset with fewer nodes than used in a memory policy
> mask] is the result of constraining a task to a cpuset that obviously
> wants access to more nodes -- it's a userspace mistake and abusing
> cpusets so that the task does not get what it expects.

Nah - I wouldn't put it that way. It's no mistake or abuse. It's just
one more example of a kernel making too few resources look sufficient
by sharing, multiplexing and virtualizing them. That's what kernels do.

--
I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <pj@sgi.com> 1.925.600.0401