—package
Parallel::Pvm;
use
strict;
use
Carp;
require
Exporter;
require
DynaLoader;
require
AutoLoader;
@ISA
=
qw(Exporter DynaLoader)
;
# Items to export into callers namespace by default. Note: do not export
# names by default without a very good reason. Use EXPORT_OK instead.
# Do not simply export all your public functions/methods/constants.
@EXPORT
=
qw
(
PVM_BYTE
PVM_CPLX
PVM_DCPLX
PVM_DOUBLE
PVM_FLOAT
PVM_INT
PVM_LONG
PVM_SHORT
PVM_STR
PVM_UINT
PVM_ULONG
PVM_USHORT
PvmAllowDirect
PvmAlready
PvmAutoErr
PvmBadMsg
PvmBadParam
PvmBadVersion
PvmCantStart
PvmDSysErr
PvmDataDefault
PvmDataFoo
PvmDataInPlace
PvmDataRaw
PvmDebugMask
PvmDontRoute
PvmDupEntry
PvmDupGroup
PvmDupHost
PvmFragSize
PvmHostAdd
PvmHostCompl
PvmHostDelete
PvmHostFail
PvmMismatch
PvmMppFront
PvmNoBuf
PvmNoData
PvmNoEntry
PvmNoFile
PvmNoGroup
PvmNoHost
PvmNoInst
PvmNoMem
PvmNoParent
PvmNoSuchBuf
PvmNoTask
PvmNotImpl
PvmNotInGroup
PvmNullGroup
PvmOk
PvmOutOfRes
PvmOutputCode
PvmOutputTid
PvmOverflow
PvmPollConstant
PvmPollSleep
PvmPollTime
PvmPollType
PvmResvTids
PvmRoute
PvmRouteDirect
PvmSelfOutputCode
PvmSelfOutputTid
PvmSelfTraceCode
PvmSelfTraceTid
PvmShowTids
PvmSysErr
PvmTaskArch
PvmTaskChild
PvmTaskDebug
PvmTaskDefault
PvmTaskExit
PvmTaskHost
PvmTaskSelf
PvmTaskTrace
PvmTraceCode
PvmTraceTid
);
$VERSION
=
'1.2.1'
;
sub
AUTOLOAD
{
# This AUTOLOAD is used to 'autoload' constants from the constant()
# XS function. If a constant is not found then control is passed
# to the AUTOLOAD in AutoLoader.
my
$constname
;
(
$constname
=
$AUTOLOAD
) =~ s/.*:://;
croak
"& not defined"
if
$constname
eq
'constant'
;
my
$val
= constant(
$constname
,
@_
?
$_
[0] : 0);
if
($! != 0)
{
if
($! =~ /Invalid/)
{
$AutoLoader::AUTOLOAD
=
$AUTOLOAD
;
goto
&AutoLoader::AUTOLOAD
;
}
else
{
croak
"Your vendor has not defined Parallel::Pvm macro $constname"
;
}
}
no
strict
'refs'
;
*$AUTOLOAD
=
sub
() {
$val
};
goto
&$AUTOLOAD
;
}
bootstrap Parallel::Pvm
$VERSION
;
# Preloaded methods go here.
# Autoload methods go after =cut, and are processed by the autosplit program.
1;
__END__
# Below is the stub of documentation for your module. You better edit it!
=head1 NAME
Parallel::Pvm - Perl extension for the Parallel Virtual Machine (PVM) Message Passing System
=head1 SYNOPSIS
use Parallel::Pvm;
=head1 DESCRIPTION
The B<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 B<rexec> or B<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 B<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 from the HTTP address http://www.epm.ornl.gov/pvm.
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!
=head2 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; B<PVM_ROOT> and B<PVM_ARCH>.
B<PVM_ROOT> points to the base of the B<PVM>
installation directory, and B<PVM_ARCH> specifies the architecture
of the computer on which B<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`
=head2 Setting up your rsh permission
In order for PVM applications to run, B<rsh> permission
has to be enabled. This involves creating a B<.rhosts>
file in your B<HOME> directory containing, for each line, the host and
account name you wish to allow remote execution privillages.
An example B<.rhosts> file to allow a PVM application to
remotely execute on the host B<onyx> and B<prata> using the
account B<edward> is shown below,
onyx edward
prata edward
=head2 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, B<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 B<hostfile>,
or dynamically within the application code itself.
The first method of configuring your virtual machine
is to use the console program B<$PVM_ROOT/lib/pvm>.
Run it from the command prompt. The console program will first add the
local host into the virtual machine and display the prompt
pvm>
To add a host, eg B<onyx>, as a node in your parallel virtual machine, simply
type
pvm> add onyx
To display the current virtual machine configuration type
pvm> conf
which will display node information pertaining to the host name,
host id, host architecture, relative speed and data format.
The console program has a number of other commands which can
be viewed by typing B<help>.
The second method of configuring your virtual machine is to use
a B<hostfile>. The B<hostfile> is simply an ASCII text file
specifing the host names of the computers to be added into your
virtual machine.
Additional options may be also be defined
for the nodes pertaining to the working directory,
execution path, login name, alternative hostname etc. A simple
example of a B<hostfile> is shown below.
* wd=$HOME/work ep=$HOME/bin
onyx
prata.nsrc.nus.sg
laksa ep=$HOME/perl5/bin
In the above example B<hostfile> we are adding the
hosts B<onyx>, B<prata.nsrc.nus.sg> and B<laksa> into the
virtual machine. We are also specifying the working
directory, B<wd>, in which we want our application
to run, and the execution path, B<ep>, in which we want PVM
to look for executables.
The B<*> 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 B<laksa> where
we have specified its execution path to be B<$HOME/perl5/bin>
instead of the B<$HOME/bin>.
The third method of configuring your virtual machine
is to call the functions B<Parallel::Pvm::addhosts> or B<Parallel::Pvm::delhosts>
within your application. You must still start your master
B<pvmd> daemon first. This can be achieved by starting
B<pvm> and typing B<quit> or simply typing
echo quit | pvm
The PVM application can then be started where
we can add the hosts B<prata> and B<laksa> by calling
Parallel::Pvm::addhosts("prata","laksa");
Or we can delete a host from our configuration by calling
Parallel::Pvm::delhosts("laksa");
PVM also provides a function, B<Parallel::Pvm::conf>, to query the configuration
of the parallel virtual machine. An example code to check the current
configuration is shown below.
($info,@conf) = Parallel::Pvm::conf ;
if ( $info == PvmOk ){
foreach $node (@conf){
print "host id = $node->{'hi_tid'}\n";
print "host name = $node->{'hi_name'}\n";
print "host architecture = $node->{'hi_arch'}\n";
print "host speed = $node->{'hi_speed'}\n";
}
}
=head2 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
$mytid = Parallel::Pvm::mytid ;
where B<$mytid> is the task id, B<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.
=head2 Spawning parallel tasks
A PVM application can spawn parallel tasks in your parallel
virtual machine. Assuming there is exists an executable called
B<client>, we can spawn four B<client> tasks in our virtual
machine by calling
($ntask,@tids) = Parallel::Pvm::spawn("client",4);
For each of the four spawned processes, the PVM system first
allocates a host node and looks for the executable in the
execuation path of that host. If the executable is found it
is started.
The task which called the B<Parallel::Pvm::spawn> is known as
the B<parent> task.
The number of B<children> tasks which are actually spawned by
B<Parallel::Pvm::spawn> is returned in the scalar B<$ntask>.
The B<@tids> array returns the task id, B<TID>, of the spawned
B<children> tasks which will be useful later for
communicating with them. A B<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 ! \n";
}else if ( $tid == PvmSysErr ){
warn "pvmd not responding ! \n";
} ...
}
}
For more sophisticated users, B<Parallel::Pvm::spawn> may be given additional
argument parameters to control how/where you want a task to be spawned.
For example, you can specifically spawn B<client> in the internet
host B<onyx.nsrc.nus.sg> by calling
Parallel::Pvm::spawn("client",1,PvmTaskHost,"onyx.nsrc.nus.sg");
Or you can spawn B<client> on host nodes only of a particular architecture,
say RS6K workstations, by calling
Parallel::Pvm::spawn("client",4,PvmTaskArch,"RS6K");
Also, if the spawned remote executable requires an argument B<argv>,
you can supply this by calling
Parallel::Pvm::spawn("client",4,PvmTaskArch,"RS6K",argv);
Note that tasks which have been spawned by using B<Parallel::Pvm::spawn>
do not need to be explicitly enrolled into the pvm system.
=head2 Exchanging messages between tasks
Messages can be sent to a task enrolled into PVM by specifying
the example code sequence
Parallel::Pvm::initsend ;
Parallel::Pvm::pack(2.345,"hello dude");
Parallel::Pvm::pack(1234);
Parallel::Pvm::send($dtid,999);
In our example we first call B<Parallel::Pvm::initsend> to initialize
the internal PVM send buffer.
We then call B<Parallel::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 B<Parallel::Pvm::send> to do the actual send to the task identifed by the B<TID>
B<$dtid>. We also label the sending message to disambiguate it with
other messages with a tag. This is done with the 999 argument in
B<Parallel::Pvm::send> function.
For the destination task, we can receive the message sent by
performing a blocking receive with the function B<Parallel::Pvm::recv>.
A code sequence for the above example on the recipent
end will be
if ( Parallel::Pvm::recv >= 0 ){
$int_t = Parallel::Pvm::unpack ;
($double_t,$str_t) = Parallel::Pvm::unpack ;
}
Note that we must unpack the message in the reverse order in which we packed
our message.
In our example B<Parallel::Pvm::recv> will receive any message sent to it.
In order to selectively receive a message, we could specify
the B<TID> of the source task and the message B<tag>. For
example,
$tag = 999;
Parallel::Pvm::recv($stid,$tag) ;
Other message passing functions that you may find useful are
B<Parallel::Pvm::psend>, B<Parallel::Pvm::trecv>, B<Parallel::Pvm::nrecv> and B<Parallel::Pvm::precv>.
=head2 Parallel I/O
Note that the file descriptors in a parent task are not
inherented in the spawned B<children> tasks unlike B<fork>.
By default any file I/O will be performed in the working
directory specified in the B<hostfile> if no
absolute path was provided for the opened file.
If no working directory is specified, the default is the
B<$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 B<tty> output, tasks which are not
started from the command prompt will have their
B<stdout> and B<stderr> directed to the file pvml.<uid>.
This may be redirected to a B<parent> task by
calling
Parallel::Pvm::catchout;
for B<stdout> or
Parallel::Pvm::catchout(stderr);
for B<stderr>. You can direct the B<stdout> or B<stderr> output
of a task to another B<TID> , other then its parent, by calling
Parallel::Pvm::setopt(PvmOutTid,$tid);
=head2 Incorporating fault tolerance
The function B<Parallel::Pvm::notify> can be used to incorporate some
fault tolerance into your PVM application.
You may use it to ask the PVM
to monitor the liveliness of a set of hosts or tasks
during the execution of a PVM application.
For example you can instrument
your application to monitor 3 tasks with B<TID> B<$task1>,
B<$task2>, and B<$task3>, by using the code segments
@monitor = ($task1,$task2,$task3);
Parallel::Pvm::notify(PvmTaskExit,999,@monitor_task);
...
if ( Parallel::Pvm::probe(-1,999) ){
$task = Parallel::Pvm::recv_notify ;
print "Oops! task $task has failed ... \n" ;
}
If either B<$task1>, B<$task2> or B<$task3>
fails, the notification will take the form of
a single message with the
tag 999. The message content will inform you of
the B<TID> of the failed task.
A similar scheme may be employed for the notification of host
failures in your parallel virtual machine.
=head2 Client/Server example
B<Client:>
use Pvm;
use File::Basename;
...
# Look for server tid and assume
# server name is 'service_provider'
@task_list = Parallel::Pvm::tasks ;
foreach $task (@task_list){
$a_out = $task->{'ti_a_out'} ;
$base = basename $a_out ;
if ( $base eq 'service_provider' )
$serv_tid = $task->{'ti_tid'} ;
}
# This is just one way (not necessarily the
# best) of getting a server tid.
# You could do the same thing by reading
# the server tid posted in a file.
...
# send request for service
Parallel::Pvm::send($serv_tid,$REQUEST);
# receive service from server
Parallel::Pvm::recv(-1,$RESPONSE);
@service_packet = Parallel::Pvm::unpack ;
...
B<Server:>
while(1){
...
if ( Parallel::Pvm::probe(-1,$REQUEST) ){
# a service request has arrived !
$bufid = Parallel::Pvm::recv ;
($info,$bytes,$tag,$stid) = Parallel::Pvm::bufinfo($bufid) ;
if ( fork == 0 ){
# fork child process to handle service
...
# provide service
Parallel::Pvm::initsend ;
Parallel::Pvm::pack(@service);
Parallel::Pvm::send($stid,$RESPONSE);
# exit child process
exit ;
}
}
...
}
=head2 PVM groups
The PVM dynamic group functions have not been ported to perl yet.
These functions provide facilities for collecting processes under
a single B<group> label, and applying aggregate operations onto
them. Examples of these functions are B<Parallel::Pvm::barrier>, B<Parallel::Pvm::reduce>,
B<Parallel::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.
=head1 FUNCTIONS
=over 4
=item B<Parallel::Pvm::addhosts>
Adds one or more host names to a parallel virtual machine. Eg.
$info = Parallel::Pvm::addhosts(@host_list) ;
=item B<Parallel::Pvm::bufinfo>
Returns information about the requested message buffer. Eg.
($info,$bytes,$tag,$tid) = Parallel::Pvm::bufinfo($bufid);
=item B<Parallel::Pvm::catchout>
Catches output from children tasks. Eg.
# Parallel::Pvm::catchout(stdout);
$bufid = Parallel::Pvm::catchout;
=item B<Parallel::Pvm::config>
Returns information about the present virtual machine configuration. Eg.
($info,@host_ref_list) = Parallel::Pvm::config ;
=item B<Parallel::Pvm::delhosts>
Deletes one or more hosts from the virtual machine. Eg.
$info = Parallel::Pvm::delhosts(@host_list);
=item B<Parallel::Pvm::exit>
Tells the local PVM daemon that the process is leaving. Eg.
$info = Parallel::Pvm::exit ;
=item B<Parallel::Pvm::freebuf>
Disposes of a message buffer. Eg.
$info = Parallel::Pvm::freebuf($bufid);
=item B<Parallel::Pvm::getopt>
Shows various libpvm options. Eg.
$val = Parallel::Pvm::getopt(PvmOutputTid);
$val = Parallel::Pvm::getopt(PvmFragSize);
=item B<Parallel::Pvm::getrbuf>
Returns the message buffer identifier for the active receive buffer. Eg.
$bufid = Parallel::Pvm::getrbuf ;
=item B<Parallel::Pvm::getsbuf>
Returns the message buffer identifier for the active send buffer. Eg.
$bufid = Parallel::Pvm::getsbuf ;
=item B<Parallel::Pvm::halt>
Shuts down the entire PVM system. Eg.
$info = Parallel::Pvm::halt ;
=item B<Parallel::Pvm::hostsync>
Gets time-of-day clock from PVM host. Eg.
($info,$remote_clk,$delta) = Parallel::Pvm::hostsync($host) ;
where B<delta> is the time-of-day equivalent to B<local_clk - remote_clk>.
=item B<Parallel::Pvm::initsend>
Clears default send buffer and specifies message encoding. Eg.
# Parallel::Pvm::initsend(PvmDataDefault) ;
$bufid = Parallel::Pvm::initsend
=item B<Parallel::Pvm::kill>
Terminates a specified PVM process.
$info = Parallel::Pvm::kill($tid);
=item B<Parallel::Pvm::mcast>
Multicast the data in the active message buffer to a set of tasks. Eg.
$info = Parallel::Pvm::mcast(@tid_list,$tag);
=item B<Parallel::Pvm::mkbuf>
Creates a new message buffer. Eg.
# Parallel::Pvm::mkbuf(PvmDataDefault);
$bufid = Parallel::Pvm::mkbuf ;
$bufid = Parallel::Pvm::mkbuf(PvmDataRaw);
=item B<Parallel::Pvm::mstat>
Returns the status of a host in the virtual machine. Eg.
$status = Parallel::Pvm::mstat($host);
=item B<Parallel::Pvm::mytid>
Returns the tid of the calling process.
$mytid = Parallel::Pvm::mytid ;
=item B<Parallel::Pvm::notify>
Requests notification of PVM events. Eg.
$info = Parallel::Pvm::notify(PvmHostDelete,999,$host_list);
# turns on notification for new host
$info = Parallel::Pvm::notify(PvmHostAdd);
# turns off notification for new host
$info = Parallel::Pvm::notify(PvmHostAdd,0);
=item B<Parallel::Pvm::nrecv>
Nonblocking receive. Eg.
# Parallel::Pvm::nrecv(-1,-1);
$bufid = Parallel::Pvm::nrecv ;
# Parallel::Pvm::nrecv($tid,-1);
$bufid = Parallel::Pvm::nrecv($tid) ;
$bufid = Parallel::Pvm::nrecv($tid,$tag) ;
=item B<Parallel::Pvm::pack>
Packs active message buffer with data. Eg.
$info = Parallel::Pvm::pack(@data_list);
=item B<Parallel::Pvm::parent>
Returns the tid of the process that spawned the calling process. Eg.
$tid = Parallel::Pvm::parent ;
=item B<Parallel::Pvm::perror>
Prints the error status of the las PVM call.
$info = Parallel::Pvm::perror($msg);
=item B<Parallel::Pvm::precv>
Receives a message directly into a buffer.
# Parallel::Pvm::precv(-1,-1);
@recv_buffer = Parallel::Pvm::precv ;
# Parallel::Pvm::precv($tid,-1);
@recv_buffer = Parallel::Pvm::precv($tid);
@recv_buffer = Parallel::Pvm::precv($tid,$tag);
Note that the current limit for the receive buffer is 100 KBytes.
=item B<Parallel::Pvm::probe>
Checks whether a message has arrived. Eg.
# Parallel::Pvm::probe(-1,-1);
$bufid = Parallel::Pvm::probe ;
# Parallel::Pvm::probe($tid,-1);
$bufid = Parallel::Pvm::probe($tid);
$bufid = Parallel::Pvm::probe($tid,$tag);
=item B<Parallel::Pvm::psend>
Packs and sends data in one call. Eg.
$info = Parallel::Pvm::psend($tid,$tag,@send_buffer);
=item B<Parallel::Pvm::pstat>
Returns the status of the specified PVM process. Eg.
$status = Parallel::Pvm::pstat($tid);
=item B<Parallel::Pvm::recv>
Receives a message. Eg.
# Parallel::Pvm::recv(-1,-1);
$bufid = Parallel::Pvm::recv ;
# Parallel::Pvm::recv($tid,-1);
$bufid = Parallel::Pvm::recv($tid) ;
$bufid = Parallel::Pvm::recv($tid,$tag);
=item B<Parallel::Pvm::recvf>
Redefines the comparison function used to accept messages. Eg.
Parallel::Pvm::recvf(\&new_foo);
=item B<Parallel::Pvm::recv_notify>
Receives the notification message initiated by B<Parallel::Pvm::notify>. This
should be preceded by a B<Parallel::Pvm::probe>. Eg.
# for PvmTaskExit and PvmHostDelete notification
if ( Parallel::Pvm::probe(-1,$notify_tag) ){
$message = Parallel::Pvm::recv_notify(PvmTaskExit) ;
}
# for PvmHostAdd notification
@htid_list = Parallel::Pvm::recv_notify(PvmHostAdd);
=item B<Parallel::Pvm::recvf_old>
Resets the comparison function for accepting messages to the
previous method before a call to B<Parallel::Pvm::recf>.
=item B<Parallel::Pvm::reg_hoster>
Registers this task as responsible for adding new PVM hosts. Eg.
$info = Parallel::Pvm::reg_hoster ;
=item B<Parallel::Pvm::reg_rm>
Registers this task as a PVM resource manager. Eg.
$info = Parallel::Pvm::reg_rm ;
=item B<Parallel::Pvm::reg_tasker>
Registers this task as responsible for starting new PVM tasks. Eg.
$info = Parallel::Pvm::reg_tasker ;
=item B<Parallel::Pvm::send>
Send the data in the active message buffer. Eg.
# Parallel::Pvm::send(-1,-1);
$info = Parallel::Pvm::send ;
# Parallel::Pvm::send($tid,-1);
$info = Parallel::Pvm::send($tid);
$info = Parallel::Pvm::send($tid,$tag);
=item B<Parallel::Pvm::sendsig>
Sends a signal to another PVM process. Eg.
use POSIX qw(:signal_h);
...
$info = Parallel::Pvm::sendsig($tid,SIGKILL);
=item B<Parallel::Pvm::setopt>
Sets various libpvm options. Eg.
$oldval=Parallel::Pvm::setopt(PvmOutputTid,$val);
$oldval=Parallel::Pvm::setopt(PvmRoute,PvmRouteDirect);
=item B<Parallel::Pvm::setrbuf>
Switches the active receive buffer and saves the previous buffer. Eg.
$oldbuf = Parallel::Pvm::setrbuf($bufid);
=item B<Parallel::Pvm::setsbuf>
Switches the active send buffer. Eg.
$oldbuf = Parallel::Pvm::setsbuf($bufid);
=item B<Parallel::Pvm::spawn>
Starts new PVM processes. Eg.
# Parallel::Pvm::spawn("compute.pl",4,PvmTaskDefault,"");
($ntask,@tid_list) = Parallel::Pvm::spawn("compute.pl",4);
($ntask,@tid_list) = Parallel::Pvm::spawn("compute.pl",4,PvmTaskHost,"onyx");
($ntask,@tid_list) = Parallel::Pvm::spawn("compute.pl",4,PvmTaskHost,"onyx",argv);
=item B<Parallel::Pvm::tasks>
Returns information about the tasks running on the virtual machine. Eg.
# Parallel::Pvm::tasks(0); Returns all tasks
($info,@task_list) = Parallel::Pvm::tasks ;
# Returns only for task $tid
($info,@task_list) = Parallel::Pvm::tasks($tid) ;
=item B<Parallel::Pvm::tidtohost>
Returns the host ID on which the specified task is running. Eg.
$dtid = Parallel::Pvm::tidtohost($tid);
=item B<Parallel::Pvm::trecv>
Receive with timeout. Eg.
# Parallel::Pvm::trecv(-1,-1,1,0); time out after 1 sec
$bufid = Parallel::Pvm::trecv ;
# time out after 2*1000000 + 5000 usec
$bufid = Parallel::Pvm::trecv($tid,$tag,2,5000);
=item B<Parallel::Pvm::unpack>
Unpacks the active receive message buffer. Eg.
@recv_buffer = Parallel::Pvm::unpack ;
=back
=head1 AUTHORS
Edward Walker, edward@nsrc.nus.sg,
National Supercomputing Research Centre, Singapore
Denis Leconte, denis_leconte@geocities.com
=head1 SEE ALSO
perl(1), pvm_intro(1PVM)
=cut