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

[Paul Jackson <pj@sgi.com>]

> Let's add a Choice C:
> 
>     Any nodemask that is passed to set_mempolicy() is saved as
>     the intent of the application in struct mempolicy.

Yes

>     All policies are effected on a contextualized per-allocation
>     basis.

"contextualized" - I guess that means converted to cpuset
relative numbering - yes.

"per-allocation" - Most of the calculation of nodemasks and
zonelists is done when memory policies change.

>     Policies such as MPOL_INTERLEAVE always get AND'd with
>     pol->cpuset_mems_allowed.

Not AND'd - Folded, as in bitmap_remap().

>     If that yields numa_no_nodes, MPOL_DEFAULT is used instead.

Not an issue with Folding.

>     Policies such as MPOL_PREFERRED are respected if the node
>     is set in pol->cpuset_mems_allowed, otherwise MPOL_DEFAULT
>     is used.

Not an issue with Folding.

>     If an application attempts to setup a memory policy for
>     an MPOL_PREFERRED node that it doesn't have access to or
>     an MPOL_INTERLEAVE nodemask that is empty when AND'd with
>     pol->cpuset_mems_allowed, -EINVAL is returned and no new
>     policy is effected.

Not issues with Folding.

>     If an application gains nodes in pol->cpuset_mems_allowed that
>     now include the nodes from MPOL_INTERLEAVE or MPOL_PREFERRED,
>     that policy is then effected once again.  Otherwise,
>     MPOL_DEFAULT is still used.

Not issues with Folding.

With folding, an application that layed out an elaborate memory
policy configuration covering say 16 nodes can run in a 4 node
cpuset, where whatever would have been on node N gets folded down
to node N % 4.

With AND'ing, such an application would find 3/4's of its fancy
memory policy configuration replaced with MPOL_DEFAULT and -EINVAL
fallbacks.


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