Using MPI MPICH in C programs on our cluster Kate Cowles 22S 295 High Performance Computing Seminar Oct 4 2007 1 20 Outline 1 Introduction MPI and MPICH 2 Basic MPI ideas 3 Examples of point to point communications 4 Collective communications 5 Resources for further study 2 20 Introduction MPI and MPICH MPI Message Passing Interface not a language but a standard for libraries of functions to enable parallelization of code written in C C or Fortran several implementations including MPICH and LAM All parallelism is explicit the programmer is responsible for correctly identifying parallelism and implementing parallel algorithms using MPI constructs The number of tasks dedicated to run a parallel program is static New tasks can not be dynamically spawned during run time MPI 2 addresses this issue 3 20 Introduction MPI and MPICH MPICH compatible with parallel linear algebra library PlaPACK doesn t work with xmpi MPICH does not include new features in MPI 2 standard 4 20 Introduction MPI and MPICH MPICH on our cluster lines in my cshrc file that make MPICH my default instead of LAM setenv MPIRUN HOME opt mpich ch p4 bin set path opt mpich chp4 bin PATH 5 20 Introduction MPI and MPICH Compiling and running C programs for MPICH compiling mpicc progname c o execname example mpicc greetingsm c o greetingsm running it mpirun np processes machinefile machinefilename execname example mpirun np 12 machinefile kc machines greetingsm 6 20 Introduction MPI and MPICH Machine file specfies nodes you want to use default MPICH machine file in opt mpich shared lists only localhost contents of example machine file for testing node11 node12 node13 node14 node15 7 20 Basic MPI ideas MPI naming conventions MPI identifiers begin with MPI rest of function name is upper and lower MPI Init MPI Comm size rest of constant name is all upper case MPI COMM WORLD MPI SUCCESS 8 20 Basic MPI ideas Communicators communicator a group of processes that can send messages to each other MPI COMM WORLD communicator predefined by MPI consists of all the processes running when program execution begins i e as many as requested with np option on mpirun rank or process id integer identifier assigned by the system to each process within a communicator when the process initializes consecutive and begin at zero used by programmer to direct different processes to do different things in single program multiple data approach Note discussion of ping c goes here 9 20 Basic MPI ideas Communication point to point communication one process sends message to one other process collective communication one to many many to one many to many blocking versus non blocking communication 10 20 Basic MPI ideas Buffering system buffer space not all MPI implementations use it holds data in transit e g if one process sends message and receiver isn t ready to receive it managed entirely by MPI applications buffer program variables managed by user user managed send buffer to set up messages for sending Note discussion of greetingsm c goes here 11 20 Basic MPI ideas Blocking and non blocking communication blocking sends and receives blocking send routine returns only when it is safe to modify the application buffer your send data for reuse blocking receive returns only after the data has arrived and is ready for use by the program non blocking sends and receives Non blocking send and receive routines return almost immediately do not wait to verify that any communication events have completed 12 20 Examples of point to point communications MPI Send and MPI Recv Blocking send and receive Arguments Buffer Program application address space that references the data that is to be sent or received In most cases this is simply the variable name that is be sent received For C programs this argument is passed by reference and usually must be prepended with an ampersand var1 Data Count Indicates the number of data elements of a particular type to be sent Data Type For reasons of portability MPI predefines its elementary data types 13 20 Examples of point to point communications Arguments continued Destination An argument to send routines that identifies receiving process by rank Source An argument to receive routines that identifies sending process by rank may be set to the wild card MPI ANY SOURCE Tag Arbitrary non negative integer assigned by the programmer to uniquely identify a message Send and receive operations should match message tags 14 20 Examples of point to point communications Arguments continued Communicator Indicates the communication context set of processes for which the source or destination fields are valid usually MPI COMM WORLD Status For a receive operation indicates the source of the message and the tag of the message In C this argument is a pointer to a predefined structure MPI Status The actual number of bytes received is obtainable from Status via the MPI Get count routine 15 20 Collective communications Collective communcations must involve all processes in the scope of a communicator types synchronization processes wait until all members of the group have reached the synchronization point data Movement broadcast scatter gather all to all collective Computation reductions one member of the group collects data from the other members and performs an operation min max add multiply etc on that data 16 20 Collective communications Scatter Note discussion of scatterrows c goes here 17 20 Collective communications Broadcast 18 20 Collective communications Reduce Note discussion of reduce c goes here 19 20 Resources for further study Resources for further study These are examples only Pacheco P S Parallel Programming with MPI 1997 Morgan Kaufman http www cs usfca edu mpi shorter version available online at ftp math usfca edu pub MPI mpi guide ps Lawrence Livermore National Laboratory MPI tutorial and examples http www llnl gov computing tutorials mpi 20 20