> I think Patrick's point is that it's not too much more expensive to do the
> syscall on Linux vs just doing the cache lookup, particularly in the
> context of a long message. And it means that upper layer protocols like
> MPI don't have to deal with caches (and since MPI implementors hate
> registration caches only slightly less than we hate MPI_CANCEL, that will
> make us happy).
Stick in a separate library then?
I don't think we want the complexity in the kernel -- I personally would
argue against merging it upstream; and given that the userspace solution
is actually faster, it becomes pretty hard to justify.