pmodels
coll: add coll_attr and comm subgroups
Pull Request Description
Add a coll_attr parameter to replace the errflag parameter in internal collective interfaces. Make the lower 8-bit of coll_attr compatible to the lower 8-bit of pt2pt attr, which will avoid extra code to translate bits such as errflags when passing from collective to point-to-point. The next 8-bit is used for subgroup indexes, enabling group collectives without extra subcomms (which are expensive to maintain). We may extend in future coll_attr for passing hints such as memory alloc kinds and algorithm choices.
Add a bcast smp_new algorithm that are similar to bcast smp but uses comm subgroups instead. Because we can construct lightweight custom subgroup, we can avoid the extra local send/recv or bcast step when root is not one of the "node roots". Instead of the node_roots_comm, we can construct a inter_group made of the actual local roots.
NOTE: the bcast smp_new is covered in the collective cvar tests.
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