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Subject: Re: [OMPI users] Debugging memory use of Open MPI
From: Eugene Loh (Eugene.Loh_at_[hidden])
Date: 2009-04-09 17:52:51

Shaun Jackman wrote:

> When running my Open MPI application, I'm seeing three processors that
> are using five times as much memory as the others when they should all
> use the same amount of memory. To start the debugging process, I would
> like to know if it's my application or the Open MPI library that's
> using the additional memory. Does anyone have any tips on calculating
> the amount of memory that Open MPI is using at a given moment in time?
> My Open MPI application makes significant use of MPI_Irecv and
> MPI_Send. Every process has exactly one MPI_Irecv request active at
> any time. When it receives a message, it handles it, possibly
> transmits a response packet using MPI_Send, and starts a new MPI_Irecv
> request. What happens if one process is slow and lags behind? Will the
> messages be buffered at the sender or the receiver? Will the messages
> be buffered at the Open MPI level or at the OS level, say in a TCP
> transmit buffer or receive buffer?

I'm no expert, but I think it's something like this:

1) If the messages are short, they're sent over to the receiver. If the
receiver does not expect them (no MPI_Irecv posted), it buffers them up.

2) If the messages are long, only a little bit is sent over to the
receiver. The receiver will take in that little bit, but until an
MPI_Irecv is posted it will not signal the sender that any more can be sent.

Are these messages being sent over TCP between nodes? How long are they?