[mpich-discuss] Strange, inconsistent behaviour with MPI_Comm_spawn

[Alexander Rast]

Still more conjecture on my part, I think I understand what's going on with
the gethostbyname failed message, but have no idea how one would go about
fixing it.

The exact message gives 'gethostbyname failed, pi at Burns (errno 1)'. I think
this is a case of the underlying implementation not being intelligent
enough to strip the username off in order to resolve the host address. In
other words, it's looking for a host named 'pi at Burns' rather than a host
Burns to which it's going to authenticate as user pi.

If you try to use the hostname without user this fails, because then it
tries to authenticate using the current username from the shell that
invoked mpiexec (in my case on what will eventually become a 'root host'
machine - an ordinary Ubuntu desktop PC) which fails because on the other
MPI hosts that username doesn't exist. I have authentication set up using
RSA keys which authenticates the local user on the 'root host' desktop PC
into the 'working user' on the remote MPI hosts, but it seems the
MPI_Comm_spawn function isn't using that authentication path.

What it looks like, is that the MPICH implementation may be such that in
order to run MPI_Comm_spawn you have to have an identical username on each
host you hope to spawn a process on. I very much *hope* this isn't the
case, because this would be exceptionally awkward, it would mean that for
each different user wanting to run MPI jobs on the remote cluster they'd
have to have their own private user and directory on each node in the
cluster.

So 1) is it possible to and 2) how would I, configure the MPI_Comm_spawn
command or associated hosts so that you can authenticate as user X on host
Y (X at Y) for each spawn host using authorized keys from your user W on
machine Z, i.e. on machine Y you have in directory
/home/X/.ssh/authorized_keys an entry for W at Z?

On Tue, Jun 13, 2017 at 5:15 PM, Alexander Rast <
alex.rast.technical at gmail.com> wrote:

> OK, some progress. After more work I determined that the bulk of the truly
> bizarre inconsistencies was down to obnoxious, intrusive behaviour in
> Ubuntu of gnome-keyring-daemon, for example see the following post:
>
> https://askubuntu.com/questions/564821/why-cant-i-
> interact-with-my-ssh-agent-e-g-ssh-add-d-doesnt-work
>
> which by the way has not been fixed as of 16.04 Ubuntu.
>
> It seems gnome-keyring-daemon is a particularly badly-behaved utility and
> doesn't help itself with multiple autoload attempts at Ubuntu startup. You
> don't know what ssh agent is actually loaded and which RSA keys it has
> cached. It's also apparently very difficult to get rid of, although there
> are ways. I got it eventually to stop loading itself. (Perhaps the MPI
> community might whinge to Ubuntu about this behaviour? Many people have
> complained about the antisocial behaviour of gnome-keyring-daemon but so
> far Ubuntu's response has been: 'we can't see why this should be considered
> a problem. And we have doubts about what you're trying to achieve')
>
> So now the problem has got to the point where there are 2 alternative
> error responses, both occurring at the MPI_Comm_spawn command. I've
> included typical error outputs for both scenarios, using the code posted
> earlier. The 2 errors occur with slightly different versions of the
> configuration file used to spawn the processes, which I'm also including.
> Obviously _2 files go together.
>
> Any thoughts now on what might be causing either of these 2 problems? I
> find the gethostbyname failed messages particularly perplexing, since I'm
> able to ssh into the machines themselves without difficulty either by name
> or IP address.
>
> On Fri, Jun 9, 2017 at 1:53 PM, Alexander Rast <
> alex.rast.technical at gmail.com> wrote:
>
>> I've reached a limit of mystification. Attempting to run an MPI
>> application using MPI_Comm_spawn from a host is resulting in bizarre,
>> inconsistent behaviour of ssh and ssh-askpass.
>>
>> What I did is, I created an RSA keypair using ssh-keygen, copied the
>> public keys into the ./ssh directories on the machines I'll be running MPI
>> on, put them in the authorized_keys file, placed all the machines in the
>> known_hosts file on the launcher host (which is starting MPI_Comm_spawn),
>> then ran eval ssh-agent and added the id_rsa file to the agent on the
>> launcher host.
>>
>> You can verify that this part of the system is working because I can use
>> ssh directly to access the worker machines that will be running the
>> application.
>>
>> But when I actually try to run the MPI application, when it gets to the
>> spawn, all sorts of wierd and wild stuff happens. Sometimes a dialogue
>> (which aggressively grabs focus) comes up asking for a password (OpenSSH
>> Authentication). Other times the same program has just said that the
>> identity/authenticity of the target machine can't be established - do I
>> want to continue? (A yes causes authentication to fail). In still other
>> cases, it appeared to open the connection but then MPI crashed saying it
>> couldn't get the host by name. (yes, every machine has the hostnames of
>> every other machine in its hosts file). And in yet another case, it seemed
>> to try to run but then crashed saying unexpected end-of-file. And so on.
>> There seems to be no rhyme or reason to the errors, I can't reproduce
>> anything, each time I try it some new and surprising behaviour comes up.
>> What's happening? Do I have to do something unusual with machine
>> configuration/environment?
>>
>> Here are the associated MPI files if anyone wants to look for errors. In
>> fact there are probably some errors in the code itself, because it's never
>> been able to be debugged (because of this wierd behaviour) but I am fairly
>> sure at least the sequence through to the spawn command is OK. All help
>> appreciated...
>>
>
>
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