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From: Daniel Rozenbaum (drozenbaum_at_[hidden])
Date: 2007-09-19 17:27:32

I'm now running the same experiment under valgrind. It's probably going to run for a few days, but interestingly what I'm seeing now is that while running under valgrind's memcheck, the app has been reporting much more of these "recv failed" errors, and not only on the server node:


If in the original run I got 3 such messages, in the valgrind'ed run I got about 45 so far, and the app still has about 75% of the work left.

I'm checking while all this is happening, and all the client processes are still running, none exited early.

I've been analyzing the debug output in my original experiment, and it does look like the server never receives any new messages from two of the clients after the "recv failed" messages appear. If my analysis is correct, these two clients ran on the same host. It might be the case then that the messages with the next tasks to execute that the server attempted to send to these two clients never reached them, or were never sent. Interesting though that there were two additional clients on the same host, and those seem to have kept working all along, until the app got stuck.

Once this valgrind experiment is over, I'll proceed to your other suggestion about the debug loop on the server side checking for any of the requests the app is waiting for being MPI_REQUEST_NULL.

Many thanks,

Jeff Squyres wrote:
On Sep 17, 2007, at 11:26 AM, Daniel Rozenbaum wrote:

What seems to be happening is this: the code of the server is  
written in
such a manner that the server knows how many "responses" it's supposed
to receive from all the clients, so when all the calculation tasks  
been distributed, the server enters a loop inside which it calls
MPI_Waitany on an array of handles until it receives all the  
results it
expects. However, from my debug prints it looks like all the clients
think they've sent all the results they could, and they're now all
sitting in MPI_Probe, waiting for the server to send out the next
instruction (which is supposed to contain a message indicating the end
of the run). So, the server is stuck in MPI_Waitany() while all the
clients are stuck in MPI_Probe().

On the server side, try putting in a debug loop and see if any of the  
requests that your app is waiting for are not MPI_REQUEST_NULL (it's  
not a value of 0 -- you'll need to compare against  
MPI_REQUEST_NULL).  If there are any, see if you can trace backwards  
to see what request it is.

I was wondering if you could comment on the "readv failed" messages  
seeing in the server's stderr:

mca_btl_tcp_frag_recv: readv failed with errno=110

I'm seeing a few of these along the server's run, with errno=110
("Connection timed out" according to the "perl -e 'die$!=errno'"  
I found in OpenMPI FAQs), and I've also seen errno=113 ("No route to
host"). Could this mean there's an occasional infrastructure  
problem? It
would be strange, as it would then seem that this particular run  
triggers it?.. Could these messages also mean that some messages got
lost due to these errors, and that's why the server thinks it still  
some results to receive while the clients think they've sent  
everything out?

That is all possible.  Sorry I missed that message in your original  
message -- it's basically a message saying that MPI_COMM_WORLD rank 0  
got a timeout from one of the peers that it shouldn't have.

You're sure that none of your processes are exiting early, right?   
You said they were all waiting in MPI_Probe, but I just wanted to  
double check that they're all still running.

Unfortunately, our error message is not very clear about which host  
it lost the connection with -- after you see that message, do you see  
incoming communications from all the slaves, or only some of them?