On 12/17/2010 6:43 PM, Sashi Balasingam wrote:
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 (...total 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