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From: Chris Reeves (chris.reeves_at_[hidden])
Date: 2007-06-21 13:09:06

Thanks for all your replies and sorry for the delay in getting back to you.

On Tue, Jun 19, 2007 at 01:40:21PM -0400, Jeff Squyres wrote:
> On Jun 19, 2007, at 9:18 AM, Chris Reeves wrote:
> > Also attached is a small patch that I wrote to work around some firewall
> > limitations on the nodes (I don't know if there's a better way to do this
> > - suggestions are welcome). The patch may or may not be relevant, but I'm
> > not ruling out network issues and a bit of peer review never goes amiss
> > in case I've done something very silly.
> From the looks of the patch, it looks like you just want Open MPI to
> restrict itself to a specific range of ports, right? If that's the
> case, we'd probably do this slightly differently (with MCA parameters
> -- we certainly wouldn't want to force everyone to use a hard-coded
> port range). Brian's also re-working some TCP and OOB issues on a /
> tmp branch right now; we'd want to wait until he's done before
> applying a similar patch.

I thought that would be the 'official', configurable way to do it. But I lack
a thorough enough understanding of how everything fits together to implement
it in that way.

> My first question is: why are you calling MPI_BARRIER? ;-)

Good question. Thinking about it, not all occurences are probably necessary. I
didn't write this code, but I will discuss this with my colleague.

> Clearly, if we're getting stuck in there, it could be a bug. Have
> you run your code through a memory-checking debugger? It's hard to
> say exactly what the problem is without more information -- it could
> be your app, it could be OMPI, it could be the network, ...
> It's a good datapoint to run with other MPI implementations, but "it
> worked with MPI X" isn't always an iron-clad indication that the new
> MPI is at fault. I'm not saying we don't have bugs in Open MPI :-)
> -- I'm just saying that I agree with you: more data is necessary.

The code is compiled with debugging turned on (with gcc's -g flag). I believe
that this does a certain degree of memory checking, but I'd have to look it up
to make sure...

Indeed. I'm not necessarily blaming OpenMPI :-p The above was merely, as you
say, an additional datapoint.

> > (gdb) where
> > #0 0x9000121c in sigprocmask ()
> > #1 0x01c46f96 in opal_evsignal_recalc ()
> > #2 0x01c458c2 in opal_event_base_loop ()
> > #3 0x01c45d32 in opal_event_loop ()
> > #4 0x01c3e6f2 in opal_progress ()
> > #5 0x01b6083e in ompi_request_wait_all ()
> > #6 0x01ec68d8 in ompi_coll_tuned_sendrecv_actual ()
> > #7 0x01ecbf64 in ompi_coll_tuned_barrier_intra_bruck ()
> > #8 0x01b75590 in MPI_Barrier ()
> Just a quick sanity check: I assume the call stack is the same on all
> processes, right? I.e., ompi_coll_tuned_barrier_intra_bruck () is
> the call right after MPI_BARRIER?

It is similar. Obviously different processes are at different points in the
loop when I attach, but the traces are similar enough. All of them have
ompi_coll_tuned_barrier_intra_bruck in the stack after MPI_Barrier.

> > What if some packets went missing on the network? Surely TCP should take
> > care of this and resend?
> What is the topology of the network that you're running on?

9 machines are physically co-located and each have a single connection to one
of two linked switches. The 10th machine is in a different part of the
building, but on the same subnet (off a different switch). All machines can
talk to each other under normal conditions.

> > As implied by my line of questioning, my current thoughts are that some
> > messages between nodes have somehow gone missing. Could this happen? What
> > could cause this? All machines are on the same subnet.
> Hmm. On a single subnet, but you need the firewall capability -- are
> they physically remote from each other, or do you just have the local
> firewalling capabilities enabled on each node?

Each node has a local firewall set up by the systems administrator, who was
persuaded to poke a 'small' (1000-port) hole in said firewall for
communication between the nodes. There are no further firewalls between the
nodes. The firewalls are there to stay.

Cheers for all your help so far,