Download Message Passing with PVM and MPI: A Comparison of Parallel Programming Environments - Prof and more Study notes Computer Science in PDF only on Docsity! Message Passing with PVM and MPI 2CMSC 818 – Alan Sussman (from J. Hollingsworth) PVM Provide a simple, free, portable parallel environment Run on everything – Parallel Hardware: SMP, MPPs, Vector Machines – Network of Workstations: ATM, Ethernet, • UNIX machines and PCs running Win32 API – Works on a heterogenous collection of machines • handles type conversion as needed Provides two things – message passing library • point-to-point messages • synchronization: barriers, reductions – OS support • process creation (pvm_spawn) 5CMSC 818 – Alan Sussman (from J. Hollingsworth) PVM Process Control Creating a process – pvm_spawn(task, argv, flag, where, ntask, tids) – flag and where provide control of where tasks are started – ntask controls how many copies are started – program must be installed on target machine Ending a task – pvm_exit – does not exit the process, just the PVM machine Info functions – pvm_mytid() - get the process task id 6CMSC 818 – Alan Sussman (from J. Hollingsworth) PVM Group Operations Group is the unit of communication – a collection of one or more processes – processes join group with pvm_joingroup(“<group name>“) – each process in the group has a unique id • pvm_gettid(“<group name>“) Barrier – can involve a subset of the processes in the group – pvm_barrier(“<group name>“, count) Reduction Operations – pvm_reduce( void (*func)(), void *data, int count, int datatype, int msgtag, char *group, int rootinst) • result is returned to rootinst node • does not block – pre-defined funcs: PvmMin, PvmMax,PvmSum,PvmProduct 7CMSC 818 – Alan Sussman (from J. Hollingsworth) PVM Performance Issues Messages have to go through PVMD – can use direct route option to prevent this problem Packing messages – semantics imply a copy – extra function call to pack messages Heterogenous Support – information is sent in machine independent format – has a short circuit option for known homogenous comm. • passes data in native format then 10CMSC 818 – Alan Sussman (from J. Hollingsworth) MPI Communicators Provide a named set of processes for communication – plus a context – system allocated unique tag All processes within a communicator can be named – numbered from 0…n-1 Allows libraries to be constructed – application creates communicators – library uses it – prevents problems with posting wildcard receives • adds a communicator scope to each receive All programs start with MPI_COMM_WORLD – Functions for creating communicators from other communicators (split, duplicate, etc.) – Functions for finding out about processes within communicator (size, my_rank, …) 11CMSC 818 – Alan Sussman (from J. Hollingsworth) Non-Blocking Point-to-point Functions Two Parts – post the operation – wait for results Also includes a poll/test option – checks if the operation has finished Semantics – must not alter buffer while operation is pending (wait returns or test returns true) 12CMSC 818 – Alan Sussman (from J. Hollingsworth) Collective Communication Communicator specifies process group to participate Various operations, that may be optimized in an MPI implementation – Barrier synchronization – Broadcast – Gather/scatter (with one destination, or all in group) – Reduction operations – predefined and user-defined • Also with one destination or all in group – Scan – prefix reductions Collective operations may or may not synchronize – Up to the implementation, so application can’t make assumptions
