Introduction to PVM

1. Introduction to PVM PVM (Parallel Virtual Machine) is a package of libraries and runtime daemons that enables building parallel apps easily and efficiently

2. Key attributes of PVM • Runs on every UNIX and WNT/W95 • Runs over most physical networks (ethernet, FDDI, Myrinet, ATM, Shared-Memory) • A heterogeneous collection of machinescan be assembled and used as a Super Computer • Programming is completely portable • The underlying machine and network is transparent to the programmer/user • Each user has his/hers own private VM

3. History of PVM • Developed at the U. of Tennessee at Knoxville and at the Oak Ridge National Lab (ORNL) • Project leader is Jack Dongarra • PVM 2.0 released in 1992 • PVM 3.3 released in June, 1994 • Last stable version is 3.3.11 • Latest version is 3.4 beta 7 available for W95 and WNT as well

4. Lets look at a sample application PVM functions are underlined: /* Basic hello world sample program. Spawns a slave and receives a string from it. */ #include <stdio.h> #include <pvm3.h> main() { int cc, tid; char buf[100]; printf("i'm t%x\n", pvm_mytid()); /* spawn 1 copy of hello_other on any machine */ cc = pvm_spawn("hello_other", (char**)0,PvmTaskDefault, "", 1, &tid); if (cc == 1) { cc = pvm_recv(-1, -1); /* receive a message from any source */ /* get info about the sender */ pvm_bufinfo(cc, (int*)0, (int*)0, &tid); pvm_upkstr(buf); printf("from t%x: %s\n", tid, buf); } else printf("can't start hello_other\n"); pvm_exit(); exit(0); }

5. pvm_mytid int pvm_mytid(); pvm_mytid() returns the tid of the calling task. Each PVM task has a unique (tid) Task ID which is assigned to it when it is created. Each host has a range of 0x40000 tids available. Thus a tid will look like 0x40003 or 0xC001a. pvm_parent int pvm_parent(); pvm_parent() returns the tid of the calling tasks parent, the task which spawned it. In the case where the task wasn’t spawned PvmNoParent (-1) is returned.

6. pvm_spawn() • int pvm_spawn(char* task, char** argv, int flag, char* where, • int ntask, int *tids) • Spawns a task which is a executable with optional command • line arguments. Can spawn several copies on several machines • depending on the flag argument. The tids of spawned tasks are • returned in tids and the number ofspawned tasks is returned • by the function. The combination flag and where define where • the tasks will be spawned: • PvmTaskDefault - spawn on any machine • PvmTaskHost - spawn on the machine named in where • PvmTaskArch - spawn on the architecture named in where • PvmTaskDebug - spawn the task in a debugger • PvmHostCompl - spawn on any machine except where

7. Receiving a message • int pvm_recv(int source, int tag) - Block until a message sent by source with tag arrives. -1 is a wildcard value that receives from any source or tag. Returns the id of the message buffer used. • int pvm_upkstr(char* str) - Unpack a string from the message buffer. • int pvm_upkint(char* ip, int size, int size) - Unpack size integers, insert them in every other stride place in ip. • int pvm_bufinfo(int buf, int* bytes, int* tag, int* tid) - Using buf the buffer id received from pvm_recv(), get the size bytes, tag and tid of the received message.

8. Lets look at the spawned app PVM functions are underlined: /* Slave part of hello world sample program. Sends a string to the task that spawned it. */ #include <pvm3.h> #define HELLO_TAG 100 main() { int ptid; char buf[100]; ptid = pvm_parent(); strcpy(buf, "hello, world from "); gethostname(buf + strlen(buf), 64); /* send string to parent */ pvm_initsend(PvmDataDefault); pvm_pkstr(buf); pvm_send(ptid, HELLO_TAG); pvm_exit(); exit(0); }

9. Sending a message • int pvm_initsend(int encoding) - Init a buffer for sending a message. Encoding defines how the data is packed, use PvmDataDefault for XDR encoding. • int pvm_pkstr(char* str) - Pack a string into the buffer. • int pvm_pkint(int *ip, int size, int stride) - Pack an array of size integers, pack every other stride integer. • int pvm_send(int target, int tag) - Send the buffer to tid target. Identify the message by tag.

10. Message Passing in PVM • Asyncronous - pvm_send() doesn’t block pvm_recv() does. Use pvm_trecv(),pvm_nrecv() and pvm_probe() for non-blocking receives. • Ordered - Messages from task A to task B are ordered and will arrive in the order sent. • Unordered - Messages from tasks A and B to task C will arrive unordered between themselves. • Deadlockable - It is very easy to deadlock an application. • Multicast - use pvm_mcast().

11. Other groups of functions • Task control - pvm_kill(), pvm_exit(). • Information - pvm_pstat(), pvm_mstat(), pvm_config(), pvm_tasks(), pvm_tidtohost(). • Host control - pvm_addhosts(), pvm_delhosts(), pvm_halt(). • Buffer manipulation - pvm_mkbuf(), pvm_freebuf(), pvm_setrbuf(), pvm_setsbuf(), pvm_getrbuf(), pvm_setsbuf().

12. PVM Daemons • Called pvmd3 or pvmd • Created on every machine added to the Virtual Machine • The first daemon created is called the “master daemon”. • The daemons are in constant communication with each other and detect failures. A failed slave daemon is deleted, if the master daemon fails the whole VM halts. • The daemons are responsible for executing most of the pvm calls. • By default all communication is performed via the daemons. It is possible to establish direct links between tasks (see pvm_setopt() ). • By default there is only one daemon per user on each machine.