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System Components

System components

• pvmd3 daemon
  • Runs on each host

  • Provides inter-host point of contact

  • Provides authentication of tasks

  • Executes processes on host

  • Provides fault tolerance

  • Message routing and source and sink for messages

• libpvm programming library
  • Contains the message passing routines and is linked to each application component

• application components
  • User's programs, containing message passing calls, which are executed as PVM tasks

PVM Terminology

• Host
  • A physical machine such as a workstation

• Virtual Machine
  • A combination of hosts running as a single concurrent resource

• Process
  • A program, data, stack, etc such as a Unix process or node program

• Task
  • A PVM process - the smallest unit of computation

• TID
  • A unique identifier, within the virtual machine, which is associated with each task

• Message
  • An ordered list of data to be sent between tasks

Message Passing

Send and receive

• Sending a message is a 3 step process
  • Initialize a buffer using the pvmfinitsend function

  • Pack the data into the buffer using pvmfpack

  • Send the contents of the buffer to another process using pvmfsend or to a number of processes (multi-cast) using pvmfmcast

• Receiving a message is a 2 step process
  • Call the blocking routine pvmfrecv or the non-blocking routine pvmfnrecv

  • Unpack the message using pvmfunpack

Message Buffers

• PVM permits a user to manage multiple buffers but
  • there is only one active send and one active receive buffer per process at any given moment

  • the packing, sending, receiving and unpacking routines only affect active buffers

  • the developer may switch between multiple buffers for message passing

Packing Data

• In C PVM supports a number of different routines for packing data this corresponds to the different types of data to be packed

• PVM only provides one FORTRAN routine (pvmfpack) for packing data. pvmfpack handles all data types

PVM Console

Functions

• PVM console is analagous to a console on any multi-tasking computer. It is used to
  • configure the virtual machine

  • start pvm tasks (including the daemon)

  • stop pvm tasks

  • receives information and error messages

Starting the console

pvm

• The console may be started and stopped multiple times on any of the hosts on which PVM is running

• The console responds with the prompt

• The console accepts commands from standard input

Configuration of PVM

conf

• The configuration of the virtual machine may be listed using the conf command

1 host, 1 data format

HOST DTID ARCH SPEED

navaho-atm 40000 SGI5 1000

• This gives a configuration of 1 host called navaho-atm , its pvmd task id, architecture and relative speed rating

add

• Additional hosts may be added using the add command

2 successful

HOST DTID

sioux-atm 100000

mohican-atm 140000

pvm> conf

3 hosts, 1 data format

HOST DTID ARCH SPEED

navaho-atm 40000 SGI5 1000

sioux-atm 100000 SGI5 1000

mohican-atm 140000 SGI5 1000

delete

• Hosts may be removed from the virtual machine using the delete command

1 successful

HOST STATUS

sioux-atm deleted

Executing a program

spawn

• The spawn command may used to execute a program from the pvm console

• This will direct the output from the program to the screen

Leaving PVM

quit, halt

• To exit from the console session while leaving all daemons and jobs running use the quit command

• To kill all pvm processes including the console, and then shut down PVM

Using a hostfile

• PVM may be started using a hostfile to define the virtual machine

• The hostfile lists the hosts in the user's virtual machine. In its simplest form it lists one host per line

• The first host listed must be the computer on which PVM is initially started

• For example

navaho-atm

sioux-atm

• Options such as userid and passwd (lo and pw) may be specified for each host

Error handling

• Generally in Fortran the completion status of a pvm routine may be checked, if the status >= 0 then the routine completed successfully, a negative value indicating that an error has occurred

• When an error occurs, a message is automatically printed that shows the task ID, function and error

• Automatic error reporting may be disabled by calling pvmfserror

• Error reports may be generated manually by calling pvmfperror

Debugging a PVM Application

• A task run by hand can be started under a serial debugger. Add PVMDEBUG to pvmfspawn arguments

• The following steps should be followed when debugging
  • Run program as a single task and debug as any other serial program

  • Run the program on a single host to eliminate network errors and errors in message passing

  • Run the program across a few hosts to locate deadlock and synchronization problems

  • Use XPVM

Fault Detection

• Pvmd will recover automatically, applications must perform their own error recovery

• A dead host will remain dead, but may be reconfigured later

• The pvmd to pvmd message passing system drives all fault detection

• Faults are triggered by retry time-out

Future Enhancements

• The next version of pvm is supposed to have
  • Greater scalability to enable 100s of hosts to be used in parallel

  • Parallel startup

  • Better debugging: libpvm to generate trace messages

  • Better high speed network utilization eg. FDDI

  • Better console program with features such as command history, command aliases and quoting