Re: cpusets vs. mempolicy and how to get interleaving

[Ethan Solomita <solo@google.com>]

[Resend -- the original email seems to have lost its first line]

     I'm concerned that there isn't an adequate mechanism available for 
an application to request NUMA interleaving in the face of cpusets and 
modifications to mems_allowed. I'm hoping for some advice.

What we want:

     Ideally, we want a task to express its preference for interleaved 
memory allocations without having to provide a list of nodes. The kernel 
will automatically round-robin amongst the task's mems_allowed.

The problem:

     At least in our environment, an independent "cpuset manager" 
process may choose to rewrite a cpuset's mems file at any time, possibly 
increasing or decreasing the number of available nodes. If 
weight(mems_allowed) is decreased, the task's MPOL_INTERLEAVE policy's 
nodemask will be shrunk to fit the new mems_allowed. If 
weight(mems_allowed) is grown, the policy's nodemask will not gain new 
nodes.

     What we want is for the task to "set it and forget it," i.e. to 
express a preference for interleaving and then never worry about NUMA 
again. If the nodemask sent via sys_mempolicy(MPOL_INTERLEAVE) served as 
a mask against mems_allowed, then we would specify an all-1s nodemask.

     I realize that this doesn't work with backwards compatibility so 
I'm looking for advice. A new policy MPOL_INTERLEAVE_ALL that doesn't 
take a nodemask argument and interleaves within mems_allowed? Any better 
suggestions?

     Thanks!
     -- Ethan

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