PVM

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PVM
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PVM - What Is It?

• Stands for Parallel Virtual Machine
• A software tool used to create and execute
  concurrent or parallel applications.
• Operates on a collection of heterogenous Unix
  computers connected by one or more networks.
• Communication accomplished by message passing

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PVM - What Is It?

• Comprised of two main components
• the PVM daemon process (pvmd3)
• library interface routines (libpvm3.a, libfpvm3.a
  libgpvm3.a)
• Public domain software package developed at Oak
  Ridge National Laboratory.

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PVM Components
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PVM Daemon (pvmd3)

• The PVM daemon (pvmd3) is a Unix process which
  oversees the operation of user processes within a
  PVM application and coordinates inter-machine PVM
  communications.

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PVM Daemon (pvmd3)

• One daemon runs on each machine configured into
  your parallel virtual machine. Other users, with
  their own parallel virtual machines, will have
  their own pvmd3s running.
• User processes communicate with each other
  through the daemons.

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PVM Libraries

• libpvm3.a - Library of C language interface
  routines. Always required.
• They contain simple subroutine calls that the
  application programmer may imbed in concurrent or
  parallel application code.

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Provide ability to

• initiate and terminate processes
• pack, send, receive and broadcast messages
• synchronize via barriers
• query and dynamically change configuration of the
  parallel virtual machine

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Libraries

• do not directly communicate to other processes.
  Instead, they send commands to the local daemon
  and receive status information back.
• Can be installed in user filespace - default is
  HOME/pvm3/lib.
• Data format conversion (XDR) performed
  automatically between hosts of different
  architectures.

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Getting Started

• Create a directory /pvm3/bin/ARCH on each
  machine for your executable components.
• create a soft link to the actual pvm3/lib
  location ( ln -s snell/pvm3/lib /pvm3/lib)
• PVM_ROOT should be set to /pvm3
• Include the appropriate PVM library routine calls
  in your code.

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PVM Routines
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Process Control Information

• Processes are identified by process IDs
• Each process must acquire a list of all the
  process IDs in the group
• Functions
• pvm_mytid
• pvm_siblings
• Must call pvm_exit() to quit

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Sending

• Initialize buffers
• Pack the message
• Send

pvm_initsend(PvmDataDefault) pvm_pkint(token,
count, stride) pvm_send(dest, msgtag)
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Receiving

• Receive the message
• Unpack

pvm_recv(src, msgtag) pvm_upkint(token, count,
stride)
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Compile Your Application Components

• Always need libpvm3.a library.
• cc -o myprog myprog.c -IPVM_ROOT/include
  -LPVM_ROOT/lib/PVM_ARCH -lpvm3
• For Dynamic Groups, also need libgpvm3.a added
  before libpvm3.a
• Make sure that executable components are located
  in /pvm3/bin/ARCH on each machine as required.

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Create your PVM hostfile

• Your PVM hostfile defines your parallel virtual
  machine. It contains the names of all desired
  machines, one per line
• Must reside only on the machine where you start
  up PVM
• The filename can be whatever you like
• Comment lines start with ""
• Precede hostname with "" if dynamically added
  later

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Other options permitted

• lo login userid
• pw password entry required
• dx daemon path if not using default
• ep executable program component paths if not
  using default /pvm3/bin/ARCH

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Create Your HOME/.rhosts file

• The .rhosts is a standard Unix "trusted hosts"
  file - permits remote login and command execution
  without password. It is not unique to PVM -
  simply used by PVM.
• Must be in your home directory
• Should be readable by user only!
• Format host.domain.name userid

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Start the Master PVM Daemon

• Start up the master (first) daemon.
• pvmd3 hostfile
• Must be started on your local machine
• Automatically starts up daemons on all other
  machines (remote) specified in your hostfile.

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Tips

• Do not run in the background if using the
  password (pw) specification in your hostfile.
• Should only have one pvmd3 running on each
  machine in your virtual machine!
• PVM console can be started after pvmd3 by typing
  "pvm". PVM console commands can then be issued.

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Execute your application

• Begin execution of your program on your local
  machine
• myprog
• What happens from here depends upon your
  application.

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Quit PVM

• Make sure all application components include a
  PVM library call of "pvmfexit(info)" or
  "pvm_exit()".
• Halting the master pvmd3 will automatically kill
  all other pvmd3s and all processes enrolled in
  this PVM.
• Running in pvm console mode use "halt" command

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If it won't die

• Abnormal terminations of PVM may leave files in
  /tmp which prevent you from restarting (known
  bug). Should delete all /tmp/pvm. files on all
  machines before restarting the master pvmd3.
• Try "clement/bin/kill-pvms" if all else fails

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Miscellaneous Hints

• The most common problem in starting PVM daemons
  is probably due to daemons already running on the
  machine(s), or the existence of a /tmp/pvmd.uid
  file. You'll need to kill old daemons and/or
  remove any /tmp/pvmd.uid files.
• The second most common problem is probably due to
  incorrect .rhost files.

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Other Problems

• Another, less frequent startup problem is due to
  .cshrc files that require interactive input or
  which display messages.

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Be Careful

• Be careful of concurrent I/O to the same files in
  a shared filesystem! For concurrent I/O (writing)
  in a shared filesystem, you may want to implement
  a scheme to assign unique names to output files.

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Your Assignment

• Specifications are on the web
• Using a Jacobi 5 point stencil, solve the 2D
  steady state temperature distribution problem on
  a 768 x 768 grid.
• Use formula 2.1 on page 34
• Report your execution times on 2, 4, and 8
  processors.