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Subject: [OMPI users] Question about oversubscribing
From: George Markomanolis (george_at_[hidden])
Date: 2012-11-04 10:05:55

Dear all,

I am trying to execute an experiment by oversubscribing the nodes. So I
have available some clusters (I can use up to 8-10 different clusters
during one execution) and I have totally around to 1300 cores. I am
executing the EP benchmark from the NAS suite which means that there are
not a lot of MPI messages, just some collective MPI calls.

The number of the MPI processes per node, depends on the available
memory of each node. Thus in the machinefile I have declared one node 13
times if I want 13 MPI processes on it. Is that correct? Giving a
machinefile of 32768 nodes when I want to execute 32768 processes, does
OpenMPI behave like there is no oversubscribing? If yes how can I give a
machinefile where there is different number of MPI processes on each
node? The maximum number of MPI processes that I have in a node is 388.

My problem is that I can execute 16384 processes but not 32768. In the
first case I need around to 3 minutes for the execution but in the
second case, even after 7 hours the benchmark does not even start. There
is no error, I am just cancelling the job by myself but I am assuming
that something is wrong because 7 hours it is too much. I have to say
that I executed the instance of 16384 processes without any problem. I
added some debug info in the benchmark and I can see that the execution
is delayed during MPI_Init, it never passes this point. For the instance
of 16384 processes I need around to 2 minutes to finish the MPI_Init
call. I am checking the memory of all the nodes and there is at least
0.5GB free memory on each node.

I know about the parameter mpi_yield_when_idle but I have read that if
there are not a lot of MPI messages will not improve the performance. I
tried though and nothing changed. I tried also the mpi_preconnect_mpi
just in case but again nothing. Could you please indicate a reason why
is this happening?

Moreover I used just one node with 48GB memory in order to execute 2048
MPI processes without any problem, of course I just had to wait a lot.

I am using OpenMPI v1.4.1 and all the clusters are 64 bit.

I execute the benchmark with the following command:
mpirun --mca pml ob1 --mca btl tcp,self --mca btl_tcp_if_exclude
ib0,lo,myri0 -machinefile machines -np 32768 ep.D.32768

Best regards,
George Markomanolis