Newsgroups: comp.parallel.pvm
From: Clark Dorman <clark@s3i.com>
Subject: Re: message queue
Organization: PSINet
Date: 17 Mar 1998 16:08:21 -0500
Message-ID: <dbtv53u0q.fsf@macarthur.s3i.com>


Mariusz Nowostawski <nowostawski@ci-1.ci.pwr.wroc.pl> writes:
> Dear readers
> 
> I have one (simple) question and I would appreciate if somebody could
> tell me wether I am right or wrong.
> 
> Let say I have 3 processes A, B and C, with tids A_ B_ and C_TID
> respectively. 
> 
> A:     	....
>  	pvm_initsend(PvmDataDefault);
> 	pvm_send(C_TID, A_MSGTAG);
> 
> B:   	....
>  	pvm_initsend(PvmDataDefault);
> 	pvm_send(C_TID, B_MSGTAG);
> 
> C:	...
> 	pvm_recv(A_TID, A_MSGTAG);
> 	pvm_recv(B_TID, B_MSGTAG);
> 
> 
> if A sends his message first, everything is fine, but if C sends its
> message first, B is blocked. Yes?

B doesn't get blocked because it is just sending.

I believe that you wanted to write: 

} if A sends his message first, everything is fine, but if B sends its
} message first, C is blocked. Yes? 

And, no, C does _not_ get blocked.  

The following programs show this.  The first is test_order.cc.  It
creates two child processes, A and B.  It starts B first, then A.
Each is sent a message tag to use and immediately send them.  A and B
are the second program, test_order_slave.cc and differ by their passed
arguments.  The results are:

elmo:toolkit (3:48pm) 46% test_order.exe
 child id is 262146
 child id is 262147
 tid_A 262147 msgtagA 2
 tid_B 262146 msgtagB 3
Terminated
elmo:toolkit (3:51pm) 47% 

I am guessing that the way that pvm queues things is that if you are
looking for any message, it takes the one off the top (earliest
arriving FIFO).  If you want a particular message, it looks in the
queue for a message that satisfies the requirements, even if it is way
down in the queue.  Just for fun, you can launch lots and lots of B's,
(put a for loop around the 'tidB = start_child( msgtagB);' line) and
it still works.

//----------------------------------------------------------------------
//  test_order.cc
//----------------------------------------------------------------------
#include <iostream.h>
#include <stdiostream.h>        // Used because pvm uses FILE
#include <unistd.h>             // for sleep()
#include "pvm3.h"

int start_child( int msgtag );

main()
{
        pvm_start_pvmd( 0, NULL, 0);

        int tidA, tidB;
        int msgtagA = 2;
        int msgtagB = 3;

        // Start child B, then child A.
        tidB = start_child( msgtagB ) ;
        sleep ( 4 ) ;
        tidA = start_child( msgtagA ) ;

        // Force a receive from A, then from B
        int recv_A = pvm_recv( tidA, msgtagA );
        cout << " tid_A " << tidA << " msgtagA " << msgtagA << endl;

        sleep( 4 );

        int recv_B = pvm_recv( tidB, msgtagB );
        cout << " tid_B " << tidB << " msgtagB " << msgtagB << endl;

        pvm_halt();
}

int  start_child( int msgtag_to_use ) 
{
        int child_tid;
        int num_started ;
        char *argv[1];
        argv[0] = new char[ 10 ];
        sprintf( argv[0], "%d", msgtag_to_use);

        int nums = pvm_spawn( "/home/dorman/gann/src/toolkit/test_order_slave.exe",
                              argv , 0, NULL, 1, &child_tid );
        cout << " child id is " << child_tid << endl;
        return child_tid;
}
        
//--- end of test_order.cc ----------------------------------------------------------
        

//----------------------------------------------------------------------
//  test_order_slave.cc
//----------------------------------------------------------------------
#include "pvm3.h"
#include <stdlib.h>

main( int argc, char *argv[])
{
        int msg_type = atoi( argv[1] );
        pvm_initsend( PvmDataDefault );
        pvm_send( pvm_parent(), msg_type );
}
//--- end of test_order_slave.cc -----------------------------------------------