Newsgroups: comp.parallel.pvm
From: bjo@fwi.uva.nl (Benno Overeinder)
Subject: Re: pvm message passing timing
Organization: FWI, University of Amsterdam
Date: 11 Aug 1995 14:41:04 +0200
Message-ID: <40fj50$61m@word.fwi.uva.nl>

eve_li@btgmax.zko.dec.com (Eve Li) writes:

>Is this really true? Does PVM really using blocking message-passing? 
>How about pvm-psend, pvm-precv? 
>The documentation says:
>	The pvm_psend routine is asynchronous.  Computation on the
>       sending processor resumes as soon as the  buffer  is  safe
>       for  reuse.  This is in contrast to synchronous communica-
>       tion, during which computation on  the  sending  processor
>       halts  until  the  matching  receive  is  executed  by the
>       receiving processor.


Lets call it blocking asynchronous communication.  Blocking because
it resumes as soon as the buffer is safe for reuse (in contrast to 
non-blocking).   Asynchronous because it does not halt until the
matching receive is executed.  This terminology seems consistent
with MPI also.

>My guess is that gettimeofday is still better when pvm-psend/pvm-precv
>are used because pvm_psend could block if there are too much data to send
>or recv. Thus, it takes more wall clock time than cpu time. 

>any comments? 
> 

It all depends on what you want to measure.  I can imagine that
if you are interested in the performance of some piece of code
you might be interested in the cpu time.  But then, you are
not measuring communication times or even idle times of tasks
waiting for message (load imbalances) which is a fairly important
thing in parallel processing.  What remains is indeed gettimeofday.

In shared environments, such as networks of workstations, it difficult 
(or may I say impossible?) to get rid of measurement disturbations by
influences of other processes.   If you have accepted this, the only 
thing that remains is to do some decent stastics to make sure you have
some confidence in the results, viz., your measurements are not a 
result of some random process called `the system'.

Benno
-- 
 Benno Overeinder			 Computer Systems Department	     
 voice: (+31) 20 525 7536		 University of Amsterdam	    
 fax:   (+31) 20 525 7490		 Kruislaan 403, 1098 SJ  Amsterdam
 e-mail: bjo@fwi.uva.nl		 	 The Netherlands

