pvm(3)

NAME

Pvm - Perl extension for the Parallel Virtual Machine
(PVM) Message Passing System

SYNOPSIS

use Pvm;

DESCRIPTION

The PVM message passing system enables a programmer to
configure a group of (possibly heterogenous) computers
connected by a network into a parallel virtual machine.
The system was developed by the University of Tennessee,
Oak Ridge National Laboratory and Emory University.

Using PVM, applications can be developed which spawns
parallel processes onto nodes in the virtual machine to
perform specific tasks. These parallel tasks can also
periodically exchange information using a set of message
passing functions developed for the system.

PVM applications have mostly been developed in the
scientific and engineering fields. However applications
for real-time and client/server systems can also be
developed. PVM simply provides a convenient way for
managing parallel tasks and communications without need
for rexec or socket level programming.

As a utility, PVM enables an organisation to leverage on
the computers already available for parallel processing.
Parallel applications can be started during non-peak hours
to utilise idle CPU cycles. Or dedicated workstation
clusters connected via a high performance network like ATM
can be used for high performance computing.

It is recommended that you read the PVM manual pages and
the book PVM: Parallel Virtual Machine, A users's guide
and tutorial for networked parallel computing. Both the
PVM system and the book can be obtained by email from
netlib@ornl.gov or anonymous ftp from netlib2.cs.utk.edu.

For the rest of this document we will provide a tutorial
introduction to developing PVM applications using perl.
The interface for some of the PVM functions have been
changed of course to give it a more perl-like feel.

Remember think perl think parallel! Good Luck!

Environment Variables

After installing PVM on your computer, there are two mandatory environment variables that have to be set in your .login or .cshrc files; PVM_ROOT and PVM_ARCH. PVM_ROOT points to the base of the PVM installation directory, and PVM_ARCH specifies the architecture of the computer on which PVM is running. An example of how this can be set for csh is shown below,: setenv PVM_ROOT /usr/local/pvm3
setenv PVM_ARCH `$PVM_ROOT/lib/pvmgetarch`
Setting up your rsh permission
In order for PVM applications to run, rsh permission has to be enabled. This involves creating a .rhosts file in your HOME directory containing, for each line, the host and account name you wish to allow remote execution privillages. An example .rhosts file to allow a PVM application to remotely execute on the host onyx and prata using the account edward is shown below,: onyx edward
prata edward
Configuring your parallel virtual machine
Parallel process management and communications is handled by a set of distributed deamons running on each of the nodes of the virtual machine. The daemon executable, pvmd, is started when a computer is added to the virtual machine. A computer can be added to the virtual machine either statically in a console program or using a hostfile, or dynamically within the application code itself.
The first method of configuring your virtual machine is to use the console program $PVM_ROOT/lib/pvm. Run it from the command prompt. The console program will first add the local host into the virtual machine and display the prompt pvm>
To add a host, eg onyx, as a node in your parallel virtual machine, simply type: pvm> add onyx
To display the current virtual machine configuration type: pvm> conf
which will display node information pertaining to the host name, host id, host architecture, relative speed and data format. The console program has a number of other commands which can be viewed by typing help.
The second method of configuring your virtual machine is to use a hostfile. The hostfile is simply an ASCII text file specifing the host names of the computers to be added into your virtual machine.
Additional options may be also be defined for the nodes pertaining to the working directory, execution path, login name, alternative hostname etc. A simple example of a hostfile is shown below.: * wd=$HOME/work ep=$HOME/bin
onyx
prata.nsrc.nus.sg
laksa ep=$HOME/perl5/bin
In the above example hostfile we are adding the hosts onyx, prata.nsrc.nus.sg and laksa into the virtual machine. We are also specifying the working directory, wd, in which we want our application to run, and the execution path, ep, in which we want PVM to look for executables.
The * in the first line defines a global option for all the hosts specified after it. We can however provide an option locally to over-ride this global option. This is seen for the host laksa where we have specified its execution path to be $HOME/perl5/bin instead of the $HOME/bin.
The third method of configuring your virtual machine is to call the functions Pvm::addhosts or Pvm::delhosts within your application. You must still start your master pvmd daemon first. This can be achieved by starting pvm and typing quit or simply typing: echo quit | pvm
The PVM application can then be started where we can add the hosts prata.nsrc.nus.sg and laksa by calling: Pvm::addhosts("prata.nsrc.nus.sg","laksa");
Or we can delete a host from our configuration by calling: Pvm::delhosts("laksa");
PVM also provides a function, Pvm::conf, to query the configuration of the parallel virtual machine. An example code to check the current configuration is shown below.: ($info,@conf) = Pvm::conf ;
if ( $info == PvmOk ){
foreach $node (@conf){
print "host id = $node->{'hi_tid'}0;
print "host name = $node->{'hi_name'}0;
print "host architecture =

$node->{'hi_arch'}0;
print "host speed = $node->{'hi_speed'}0;

}; }
Enrolling a task into PVM
A task has to expilictly enroll into PVM in order for it to be known by other PVM tasks. This can often be done by the call $mytid = Pvm::mytid ;
where $mytid is the task id, TID, assigned by the PVM system to the calling process. Note however that calling any PVM function in a program will also enroll it into the system.
Spawning parallel tasks
A PVM application can spawn parallel tasks in your parallel virtual machine. Assuming there is exists an executable called client, we can spawn four client tasks in our virtual machine by calling: ($ntask,@tids) = Pvm::spawn("client",4);
For each of the four spawned processes, the PVM system first allocates a host node and looks for the executable in the execuation path of that host. If the executable is found it is started.
The task which called the Pvm::spawn is known as the parent task. The number of children tasks which are actually spawned by Pvm::spawn is returned in the scalar $ntask. The @tids array returns the task id, TID, of the spawned children tasks which will be useful later for communicating with them. A TID < 0 indicates a task failure to spawn and can be used to determine the nature of the problem. Eg.: foreach $tid (@tids){
if ( $tid < 0 ){
if ( $tid == PvmNoMem )
warn "no memory ! 0;

}else if ( $tid == PvmSysErr ){

warn "pvmd not responding ! 0;

} ...

}; }
For more sophisticated users, Pvm::spawn may be given additional argument parameters to control how/where you want a task to be spawned. For example, you can specifically spawn client in the internet host <onyx.nsrc.nus.sg> by calling: Pvm::spawn("client",1,Pvm; TaskHost,"onyx.nsrc.nus.sg");
Or you can spawn client on host nodes only of a particular architecture, say RS6K workstations, by calling: Pvm::spawn("client",4,PvmTaskArch,"RS6K");
Note that tasks which have been spawned by using Pvm::spawn do not need to be explicitly enrolled into the pvm system.
Exchanging messages between tasks
Messages can be sent to a task enrolled into PVM by specifying the example code sequence: Pvm::initsend ;
Pvm::pack(2.345,"hello dude");
Pvm::pack(1234);
Pvm::send($dtid,999);
In our example we first call Pvm::initsend to initialize the internal PVM send buffer. We then call Pvm::buffer to fill this buffer with a double (2.345), , a string ("hello dude"), and an integer (1234). Having filled the send buffer with the data that is to be sent, we call Pvm::send to do the actual send to the task identifed by the TID $dtid. We also label the sending message to disambiguate it with other messages with a tag. This is done with the 999 argument in Pvm::send function.
For the destination task, we can receive the message sent by performing a blocking receive with the function Pvm::recv. A code sequence for the above example on the recipent end will be: if ( Pvm::recv >= 0 ){
$int_t = Pvm::unpack ;
($double_t,$str_t) = Pvm::unpack ;; }
Note that we must unpack the message in the reverse order in which we packed our message. In our example Pvm::recv will receive any message sent to it. In order to selectively receive a message, we could specify the TID of the source task and the message tag. For example,: $tag = 999;
Pvm::recv($stid,$tag) ;
Other message passing functions that you may find useful are Pvm::psend, Pvm::trecv, Pvm::nrecv and Pvm::precv.
Parallel I/O
Note that the file descriptors in a parent task are not inherented in the spawned children tasks unlike fork. By default any file I/O will be performed in the working directory specified in the hostfile if no absolute path was provided for the opened file. If no working directory is specified, the default is the $HOME directory. For directories which are not NFS mounted, this would mean that each task performs its own separate I/O.
In the case of tty output, tasks which are not started from the command prompt will have their stdout and stderr directed to the file pvml.<uid>. This may be redirected to a parent task by calling: Pvm::catchout;
for stdout or: Pvm::catchout(stderr);
for stderr. You can direct the stdout or stderr output of a task to another TID , other then its parent, by calling: Pvm::setopt(PvmOutTid,$tid);
Incorporating fault tolerance
The function Pvm::notify can be used to incorporate some fault tolerance into your PVM application. You may use it to ask the PVM to monitor the liveliness of a set of hosts or tasks during the execution of a PVM application. For example you can instrument your application to monitor 3 tasks with TID $task1, $task2, and $task3, by using the code segments: @monitor = ($task1,$task2,$task3);
Pvm::notify(PvmTaskExit,999,@monitor_task);
...; if ( Pvm::probe(-1,999) ){
$task = Pvm::recv_notify ;
print "Oops! task $task has failed ... 0 ;; }
If either $task1, $task2 or $task3 fails, the notification will take the form of a single message with the tag 999. The message content will inform you of the TID of the failed task.
A similar scheme may be employed for the notification of host failures in your parallel virtual machine.
Client/Server example
Client:: use Pvm;
use File::Basename;
...; # Look for server tid and assume
# server name is 'service_provider'; @task_list = Pvm::tasks ;
foreach $task (@task_list){
$a_out = $task->{'ti_a_out'} ;
$base = basename $a_out ;
if ( $base eq 'service_provider' )
$serv_tid = $task->{'ti_tid'} ;

}

# This is just one way (not necessarily the
# best) of getting a server tid.
# You could do the same thing by reading
# the server tid posted in a file.

...

# send request for service
Pvm::send($serv_tid,$REQUEST);

# receive service from server
Pvm::recv(-1,$RESPONSE);
@service_packet = Pvm::unpack ;
...
Server:: while(1){
...

if ( Pvm::probe(-1,$REQUEST) ){

# a service request has arrived !
$bufid = Pvm::recv ;
($info,$bytes,$tag,$stid) = Pvm::bufin

fo($bufid) ;

if ( fork == 0 ){
# fork child process to handle service
...

# provide service
Pvm::initsend ;
Pvm::pack(@service);
Pvm::send($stid,$RESPONSE);

# exit child process
exit ;

}

}
...; }
PVM groups
The PVM dynamic group functions have not been ported to perl yet. These functions provide facilities for collecting processes under a single group label, and applying aggregate operations onto them. Examples of these functions are Pvm::barrier, Pvm::reduce, Pvm::bcast etc. One of our concerns is that these group functions may be changed or augmented in the future releases of PVM 3.4*. A decision for porting the group functions will be made after PVM 3.4 has been released.

FUNCTIONS

Pvm::addhosts Adds one or more host names to a parallel virtual machine. Eg.: $info = Pvm::addhosts(@host_list) ;
Pvm::bufinfo Returns information about the requested message buffer. Eg.: ($info,$bytes,$tag,$tid) = Pvm::bufinfo($bufid);
Pvm::catchout Catches output from children tasks. Eg.: # Pvm::catchout(stdout);
$bufid = Pvm::catchout;
Pvm::config Returns information about the present virtual machine configuration. Eg.: ($info,@host_ref_list) = Pvm::config ;
Pvm::delhosts Deletes one or more hosts from the virtual machine. Eg.: $info = Pvm::delhosts(@host_list);
Pvm::exit Tells the local PVM daemon that the process is leaving. Eg.: $info = Pvm::exit ;
Pvm::freebuf Disposes of a message buffer. Eg.: $info = Pvm::freebuf($bufid);
Pvm::getopt Shows various libpvm options. Eg.: $val = Pvm::getopt(PvmOutputTid);
$val = Pvm::getopt(PvmFragSize);
Pvm::getrbuf Returns the message buffer identifier for the active receive buffer. Eg.: $bufid = Pvm::getrbuf ;
Pvm::getsbuf Returns the message buffer identifier for the active send buffer. Eg.: $bufid = Pvm::getsbuf ;
Pvm::halt Shuts down the entire PVM system. Eg.: $info = Pvm::halt ;
Pvm::hostsync Gets time-of-day clock from PVM host. Eg.: ($info,$remote_clk,$delta) = Pvm::hostsync($host); ;
where delta is the time-of-day equivalent to local_clk remote_clk.
Pvm::initsend Clears default send buffer and specifies message encoding. Eg.: # Pvm::initsend(PvmDataDefault) ;
$bufid = Pvm::initsend
Pvm::kill Terminates a specified PVM process.: $info = Pvm::kill($tid);
Pvm::mcast Multicast the data in the active message buffer to a set of tasks. Eg.: $info = Pvm::mcast(@tid_list,$tag);
Pvm::mkbuf Creates a new message buffer. Eg.: # Pvm::mkbuf(PvmDataDefault);
$bufid = Pvm::mkbuf ;; $bufid = Pvm::mkbuf(PvmDataRaw);
Pvm::mstat Returns the status of a host in the virtual machine. Eg.: $status = Pvm::mstat($host);
Pvm::mytid Returns the tid of the calling process.: $mytid = Pvm::mytid ;
Pvm::notify Requests notification of PVM events. Eg.: $info = Pvm::notify(PvmHostDelete,999,$host_list);
Pvm::nrecv Nonblocking receive. Eg.: # Pvm::nrecv(-1,-1);
$bufid = Pvm::nrecv ;; # Pvm::nrecv($tid,-1);
$bufid = Pvm::nrecv($tid) ;; $bufid = Pvm::nrecv($tid,$tag) ;
Pvm::pack Packs active message buffer with data. Eg.: $info = Pvm::pack(@data_list);
Pvm::parent Returns the tid of the process that spawned the calling process. Eg.: $tid = Pvm::parent ;
Pvm::perror Prints the error status of the las PVM call.: $info = Pvm::perror($msg);
Pvm::precv Receives a message directly into a buffer.: # Pvm::precv(-1,-1);
@recv_buffer = Pvm::precv ;; # Pvm::precv($tid,-1);
@recv_buffer = Pvm::precv($tid);; @recv_buffer = Pvm::precv($tid,$tag);
Note that the current limit for the receive buffer is 100 KBytes.
Pvm::probe Checks whether a message has arrived. Eg.: # Pvm::probe(-1,-1);
$bufid = Pvm::probe ;; # Pvm::probe($tid,-1);
$bufid = Pvm::probe($tid);; $bufid = Pvm::probe($tid,$tag);
Pvm::psend Packs and sends data in one call. Eg.: $info = Pvm::psend($tid,$tag,@send_buffer);
Pvm::pstat Returns the status of the specified PVM process. Eg.: $status = Pvm::pstat($tid);
Pvm::recv Receives a message. Eg.: # Pvm::recv(-1,-1);
$bufid = Pvm::recv ;; # Pvm::recv($tid,-1);
$bufid = Pvm::recv($tid) ;; $bufid = Pvm::recv($tid,$tag);
Pvm::recvf Redefines the comparison function used to accept messages. Eg.: Pvm::recvf(new_foo);
Pvm::recv_notify Receives the notification message initiated by Pvm::notify. This should be preceded by a Pvm::probe. Eg.: if ( Pvm::probe(-1,$notify_tag) ){
$message = Pvm::recv_notify ;; }
Pvm::recvf_old Resets the comparison function for accepting messages to the previous method before a call to Pvm::recf.
Pvm::reg_hoster Registers this task as responsible for adding new PVM hosts. Eg.: $info = Pvm::reg_hoster ;
Pvm::reg_rm Registers this task as a PVM resource manager. Eg.: $info = Pvm::reg_rm ;
Pvm::reg_tasker Registers this task as responsible for starting new PVM tasks. Eg.: $info = Pvm::reg_tasker ;
Pvm::send Send the data in the active message buffer. Eg.: # Pvm::send(-1,-1);
$info = Pvm::send ;; # Pvm::send($tid,-1);
$info = Pvm::send($tid);; $info = Pvm::send($tid,$tag);
Pvm::sendsig Sends a signal to another PVM process. Eg.: use POSIX qw(:signal_h);
...; $info = Pvm::sendsig($tid,SIGKILL);
Pvm::setopt Sets various libpvm options. Eg.: $oldval=Pvm::setopt(PvmOutputTid,$val);; $oldval=Pvm::setopt(PvmRoute,PvmRouteDirect);
Pvm::setrbuf Switches the active receive buffer and saves the previous buffer. Eg.: $oldbuf = Pvm::setrbuf($bufid);
Pvm::setsbuf Switches the active send buffer. Eg.: $oldbuf = Pvm::setsbuf($bufid);
Pvm::spawn Starts new PVM processes. Eg.: # Pvm::spawn("compute.pl",4,PvmTaskDefault,"");
($ntask,@tid_list) = Pvm::spawn("compute.pl",4);; ($ntask,@tid_list) = Pvm::spawn("com; pute.pl",4,PvmTaskHost,"onyx");
Pvm::tasks Returns information about the tasks running on the virtual machine. Eg.: # Pvm::tasks(0); Returns all tasks
($info,@task_list) = Pvm::tasks ;; # Returns only for task $tid
($info,@task_list) = Pvm::tasks($tid) ;
Pvm::tidtohost Returns the host ID on which the specified task is running. Eg.: $dtid = Pvm::tidtohost($tid);
Pvm::trecv Receive with timeout. Eg.: # Pvm::trecv(-1,-1,1,0); time out after 1 sec
$bufid = Pvm::trecv ;; # time out after 2*1000000 + 5000 usec
$bufid = Pvm::trecv($tid,$tag,2,5000);
Pvm::unpack Unpacks the active receive message buffer. Eg.: @recv_buffer = Pvm::unpack ;

AUTHOR

Edward Walker, edward@nsrc.nus.sg, National Supercomputing
Research Centre

docs.sk

comprehensive documentation repository

In section