Yo all A recent email thread on the devel list involved (in part) the question of hostname resolution. [Note: I have a fix for the localhost problem described in that thread - just need to chase down a memory corruption problem, so it won't come into the trunk until next week]
This is a problem that has troubled us since the beginning, and we have gone back-and-forth on solutions. Rather than just throwing another code change into the system, Jeff and I thought it might be a good idea to seek input from the community. The problem is that our system requires a consistent way of identifying nodes so we can tell if, for example, we already have a daemon on that node. We currently do that via a string hostname. This appears to work just fine in managed environments as the allocators are (usually?) consistent in how they name a node. However, users are frequently not consistent, which causes a problem. For example, users can create a hostfile entry for "foo.bar.net", and then put "-host foo" on their command line. In Open MPI, these will be treated as two completely separate nodes. In the past, we attempted to solve this by actually resolving every name provided to us. However, resolving names of remote hosts can be a very expensive function call, especially at scale. One solution we considered was to only do this for non-managed environments - i.e., when provided names in a hostfile or via -host. This was rejected on the grounds that it penalized people who used those mechanisms and, in many cases, wasn't necessary because users were careful to avoid ambiguity. But that leaves us with an unsolved problem that can cause Open MPI to behave in unexpected ways, including possibly hanging. Of course, we could just check names for matches in that first network name field - this would solve the "foo" vs "foo.bar.net" problem, but creates a vulnerability (what if we have both "foo.bar.net" and "foo.no-bar.net" in our hostfile?) that may or may not be acceptable (I'm sure it is at least uncommon for an MPI app to cross subnet boundaries, but maybe someone is really doing this in some rsh-based cluster). Or we could go back to fully resolving names provided via non-managed channels. Or we just tell people that "you *must* be consistent in how you identify nodes". Or....? Any input would be appreciated. Ralph
