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Subject: Re: [OMPI users] Call to MPI_Test has large time-jitter
From: Tim Prince (n8tm_at_[hidden])
Date: 2010-12-18 07:00:36

On 12/17/2010 6:43 PM, Sashi Balasingam wrote:
> Hi,
> I recently started on an MPI-based, 'real-time', pipelined-processing
> application, and the application fails due to large time-jitter in
> sending and receiving messages. Here are related info -
> 1) Platform:
> a) Intel Box: Two Hex-core, Intel Xeon, 2.668 GHz ( of 12 cores),
> b) OS: SUSE Linux Enterprise Server 11 (x86_64) - Kernel \r (\l)
> c) MPI Rev: (OpenRTE) 1.4, (...Installed OFED package)
> d) HCA: InfiniBand: Mellanox Technologies MT26428 [ConnectX IB QDR,
> PCIe 2.0 5GT/s] (rev a0)
> 2) Application detail
> a) Launching 7 processes, for pipelined processing, where each process
> waits for a message (sizes vary between 1 KBytes to 26 KBytes),
> then process the data, and outputs a message (sizes vary between 1
> KBytes to 26 KBytes), to next process.
> b) MPI transport functions used : "MPI_Isend", MPI_Irecv, MPI_Test.
> i) For Receiving messages, I first make an MPI_Irecv call, followed
> by a busy-loop on MPI_Test, waiting for message
> ii) For Sending message, there is a busy-loop on MPI_Test to ensure
> prior buffer was sent, then use MPI_Isend.
> c) When the job starts, all these 7 process are put in High priority
> mode ( SCHED_FIFO policy, with priority setting of 99).
> The Job entails an input data packet stream (and a series of MPI
> messages), continually at 40 micro-sec rate, for a few minutes.
> 3) The Problem:
> Most calls to MPI_Test (...which is non-blocking) takes a few
> micro-sec, but around 10% of the job, it has a large jitter, that vary
> from 1 to 100 odd millisec. This causes
> some of the application input queues to fill-up and cause a failure.
> Any suggestions to look at on the MPI settings or OS config/issues
> will be much appreciated.
I didn't see anything there about your -mca affinity settings. Even if
the defaults don't choose optimum mapping, it's way better than allowing
them to float as you would with multiple independent jobs running.

Tim Prince