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Subject: Re: [OMPI devel] Simple program (103 lines) makes Open-1.4.3 hang
From: Eugene Loh (eugene.loh_at_[hidden])
Date: 2010-11-23 17:45:10


Sébastien Boisvert wrote:
Le mardi 23 novembre 2010 à 16:07 -0500, Eugene Loh a écrit :
  
Sébastien Boisvert wrote:
    
Case 1: 30 MPI ranks, message size is 4096 bytes

File: mpirun-np-30-Program-4096.txt
Outcome: It hangs -- I killed the poor thing after 30 seconds or so.
      
4096 is rendezvous.  For eager, try 4000 or lower.
    
According to ompi_info, the threshold is 4096, not 4000, right ?
  
Right.
"btl_sm_eager_limit: Below this size, messages are sent "eagerly" --
that is, a sender attempts to write its entire message to shared buffers
without waiting for a receiver to be ready. Above this size, a sender
will only write the first part of a message, then wait for the receiver
to acknowledge its ready before continuing. Eager sends can improve
performance by decoupling senders from receivers."

source:
http://www.open-mpi.org/faq/?category=sm#more-sm

It should say "Below this size or equal to this size" instead of "Below
this size" as ompi_info says. ;)
  
Well, I guess it should say:

If message data plus header information fits within this limit, the message is sent "eagerly"...

I guess I'll fix it.  (I suspect I wrote it in the first place.  Sigh.)