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Subject: Re: [OMPI users] Issue with : mca_oob_tcp_peer_recv_connect_ack on SGI Altix
From: Ralph Castain (rhc_at_[hidden])
Date: 2010-12-15 15:24:14

On Dec 15, 2010, at 1:11 PM, Gilbert Grosdidier wrote:

> Ralph,
> I am not using the TCP BTL, only OPENIB one. Does this change the number of sockets in use per node, please ?

I believe the openib btl opens sockets for connection purposes, so the count is likely the same. An IB person can confirm that...

> But I suspect the ORTE daemons are communicating only through TCP anyway, right ?


> Also, is there anybody in the OpenMPI team using an SGI Altix cluster with a high number of nodes
> (1k nodes, ie 8k cores) that I could ask him (her) about the right setup ?

Not sure, but 1k nodes isn't particularly large - we launch much larger clusters without problem.

My guess is that you have some zombie procs running that are messing up the communication. You said you had undergone a lot of crashed jobs that might have left zombies in their wake. Can you clean those up?

> Thanks, Best, G.
> Le 15/12/2010 21:03, Ralph Castain a écrit :
>> On Dec 15, 2010, at 12:30 PM, Gilbert Grosdidier wrote:
>>> Bonsoir Ralph,
>>> Le 15/12/2010 18:45, Ralph Castain a écrit :
>>>> It looks like all the messages are flowing within a single job (all three processes mentioned in the error have the same identifier). Only possibility I can think of is that somehow you are reusing ports - is it possible your system doesn't have enough ports to support all the procs?
>>> Seems there is on every worker node a range of almost 30k ports available:
>>>> ssh r33i0n0 cat /proc/sys/net/ipv4/ip_local_port_range
>>> 32768 61000
>>> This is AFAIK the only way I can get info about this.
>>> Are these 30k ports this enough ?
>> Depends on how many nodes there are in your system.
>>> Question is : is OpenMPI opening ports from every node towards every other node ?
>>> In such a case I could figure out why it is going to to lacking ports when
>>> I increase the number of nodes.
>> Yes - in two ways:
>> 1. each ORTE daemon opens a port to every other daemon in the system. Thus, you need at least M ports if your job is running across M nodes
>> 2. each MPI process will open a direct port to any other MPI process that it communicates with. So if you have N processes on a node, and they only communicate to the 8 nearest neighbor nodes (each of which have N processes), and you are using the TCP btl, then you will consume an additional 8*N*N sockets on each node.
>>> But: is there a possibility (mca param ?) to prevent OpenMPI to open so many ports ?
>>> Indeed, apart from rank 0 node, every MPI process will need to communicate with ONLY
>>> the 8 (nearest) neighbour nodes. So, there should be a switch somewhere telling OpenMPI
>>> to open a port ONLY when needed, but I did not find it among ompi_info stuff ;-)
>> It always only opens a port when something tries to communicate - we never open ports in advance.
>>> Which one is it ?
>>> Thanks, Best, G.