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Subject: [OMPI users] Very high latency with openib btl
From: Maxime Boissonneault (maxime.boissonneault_at_[hidden])
Date: 2013-02-14 11:50:31


Hi,
I have a strange case here. The application is "plink"
(http://pngu.mgh.harvard.edu/~purcell/plink/download.shtml). The
computation/communication pattern of the application is the following :

1- MPI_Init
2- Some single rank computation
3- MPI_Bcast
4- Some single rank computation
5- MPI_Barrier
6- rank 0 sends data to each other rank with MPI_Ssend, one rank at a time.
6- other ranks use MPI_Recv
7- Some single rank computation
8- other ranks send result to rank 0 with MPI_Ssend
8- rank 0 receives data with MPI_Recv
9- rank 0 analyses result
10- MPI_Finalize

The amount of data being sent is of the order of the kilobytes, and we
have IB.

The problem we observe is in step 6. I've output dates before and after
each MPI operation. With the openib btl, the behavior I observe is that :
- rank 0 starts sending
- rank n receives almost instantly, and MPI_Recv returns.
- rank 0's MPI_Ssend often returns _minutes_.

It looks like the acknowledgement from rank n takes minutes to reach
rank 0.

Now, the tricky part is that if I disable the openib btl to use instead
tcp over IB, there is no such latency and the acknowledgement comes back
within a fraction of a second. Also, if rank 0 and rank n are on the
same node, the acknowledgement is also quasi-instantaneous (I guess it
goes through the SM btl instead of openib).

I tried to reproduce this in a simple case, but I observed no such
latency. The duration that I got for the whole communication is of the
order of milliseconds.

Does anyone have an idea of what could cause such very high latencies
when using the OpenIB BTL ?

Also, I tried replacing step 6 by explicitly sending a confirmation :
- rank 0 does MPI_Isend to rank n followed by MPI_Recv from rank n
- rank n does MPI_Recv from rank 0 followed by MPI_Isend to rank 0

In this case also, rank n's MPI_Isend executes quasi-instantaneously,
and rank 0's MPI_Recv only returns a few minutes later.

Thanks,

Maxime Boissonneault