Newsgroups: comp.parallel.mpi
From: nmm1@cus.cam.ac.uk (Nick Maclaren)
Subject: Re: causality violation
Organization: University of Cambridge, England
Date: 17 Dec 1996 18:09:23 GMT
Message-ID: <596nkj$54n@lyra.csx.cam.ac.uk>

In article <596j00$ubg@www.univie.ac.at>, hejc@apap2.pap.univie.ac.at writes:
|> 
|>  I often use 'nusphot' or 'VAMPIR' to investigate my
|> MPI programs. Two-point communications are displayed with
|> arrows, pointing from the sender process to the receiver
|> process. 
|>  In some of my programs, the receiving of the message occurs
|> before the sending, and the arrows are pointing to the past, which
|> should be impossible.
|> 
|>  Can somebody explain me what's the origin of this 'causality
|> violation'?
|>  Is it a failure of the tracefile generation, or a failure
|> of the MPI program, and what conclusions can be drawn from the
|> occurrence of these time-reversed messages?

Well, before chasing up such tricky problems, check that the clocks
on those systems are synchronised!  If you have difficulty doing
this, ask me for my SNTP program which can be used to check the
time difference.


Nick Maclaren,
University of Cambridge Computer Laboratory,
New Museums Site, Pembroke Street, Cambridge CB2 3QG, England.
Email:  nmm1@cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679

