Linux compatible setaffinity.

[Robert Watson]

On Fri, 11 Jan 2008, Andrew Gallatin wrote:

> I'm somewhat surprised that this has not hit the tree yet.  What happened? 
> Wasn't the consensus that it was a good thing?

I think Jeff just got busy with other stuff.

> FWIW, I was too busy to reply at the time, but I agree that the Apple 
> interface is nice.  However, sometimes one needs a hard CPU binding 
> interface like this one, and I don't see any reason to defer adding this 
> interface in favor of the Apple one, since they are somewhat orthogonal. 
> I'd be strongly in favor of having a hard CPU binding interface.

The Apple API is nice in terms of capabilities, but we wouldn't be able to use 
it directly as it Mach-esque (as I understand it).  Of course, Jeff's 
implementation of the Linux API doesn't actually fully implement the API (it 
doesn't support constraining the CPU set vs. binding to one CPU, and the patch 
as-provided didn't support querying the binding).  I agree I'd like to see if 
in the tree, if only because it would let me eliminate local hacks I have that 
do the same thing, but we should think about other interfaces that are more 
expressive in the longer term.

For example, one thing I like about the Apple interface is the ability to 
specify general strategies for affinity rather than specific affinities -- 
"these threads like to be together, but they don't mind where that is". 
Likewise, the Solaris facility to be able to change a CPU set and have all the 
things pinned to it follow the centrally-administered set is a nice match for 
our concept of Jail.  Finally, if we do want it to work well with Jail, and we 
want Jails to be able to be pinned to sets of CPUs, we also need a nested 
concept of how to handle affinity, in the event that the set of CPUs a Jail is 
running on changes, in which case perhaps you want relative numbering within 
the jail, or some other similar notion.

Sounds like a nice whiteboard session at the BSDCan developer summit...

Robert N M Watson
Computer Laboratory
University of Cambridge