CS 591xOverview of MPI-2

Major Features of MPI-2Superset of MPI-1Parallel IO (previously discussed)Standard Process StartupDynamic Process ManagementRemote Memory Access

MPI-2MPI-1 includes no specifications for a process executor Left to individual implementations usually “mpirun” even mpirun can vary across

implementations options, parameters, keywords can be

different

MPI-2MPI-2 includes a recommendation for a standard method to start MPI processesThe result –mpiexecmpiexec arguments and parameters have standard meaningstandard = portable

mpiexec arguments -n [numprocesses]

number of processes requested (like –n in mpirun)

mpiexec –n 12 myprog

-soft [minprocesses] start job with minprocesses processes if –

n processes are not available mpiexec –n 12 –soft 6 myprog

mpiexec arguments -soft [n:m]

a soft request can be a range mpiexec –n 12 –soft 4:12 myprog

-host [hostname] requests execution on a specific host mpiexec –n 4 –host node4 myprog

-arch [archname] start the job on a specific architecture

mpiexec arguments-file [filename]

requests job to run per specifications contained in filename

mpiexec –file specfile supports the execution of multiple

executables

Remote Memory AccessRecall that in MPI-1- message passing is essentially a push

operation the sender has to initiate the

communications, or actively participate in the

communication operation (collective communications)

communications is symetrical

Remote Memory AccessHow would you handle a situation where: process x decides that it needs the

value in variable a in process y… … and process y does not initiate an

communication operation

Remote Memory AccessMPI-2 has the answer… Remote Memory Accessallows a process to initial and carryout an asymmetrical communications operation……assuming the processes have setup the appropriate objects windows

Remote Memory Accessint MPI_Win_create(

void* var,MPI_Aint size,int disp_unit,MPI_Info info,MPI_Comm comm,MPI_Win* win)

Remote Memory Accessvar – the variable to appear in the windowsize – the size of the vardisp_units – displacement unitsinfo – key-value pairs to express “hints” to

MPI-2 on how to do the Win_createcomm – the communicator that can share

the windowwin – the name of the window object

Remote Memory Accessint MPI_Win_fence(

int assert,MPI_Win win)

assert – usually 0win - the name of the window

Remote Memory Accessint MPI_Get(

void* var,int count,MPI_Datatype datatype,int target_rank,MPI_Aint displacement,int target_count,MPI_Datatype target_datatype,MPI_Win win)

Remote Memory Accessint MPI_Win_Free(

MPI_Win*win)

Remote Memory Accessint MPI_Accumulate(

void* var,int count,MPI_Datatype datatype,int target_rank,MPI_Aint displace,int target_count,MPI_Datatype target_datatype,MPI_Op operation,MPI_Win win)

Remote Memory AccessMPI_Init(&argc, &argv);MPI_Comm_size(MPI_COMM_WORLD, &nprocs);MPI_Comm_rank(MPI_COMM_WORLD, &myrank);

if(myrank==0) {MPI_Win_create(&n, sizeof(int), 1, MPI_INFO_NULL, MPI_WORLD_COMM, &nwin); }

else {MPI_Win_create(MPI_BOTTOM, 0, 1, MPI_INFO_NULL, MPI_WORLD_COMM, &nwin); }

………….

Remote Memory AccessMPI_Win_fence(0, nwin);if (myrank != 0)

MPI_Get(&n, 1, MPI_INT, 0, 0, 1, MPI_INT, nwin);

MPI_Win_Fence(0, nwin);

Remote Memory AccessBTW--- there is a MPI_Put also

Dymanic Process Management

In MPI-1, recall that- process creation is static all processes in the job are created

when the job initializes the number of processes in the job

never vary as job execution progresses

Dynamic Process Management

MPI-2 allow the creation of new processes within the application—called spawninghelps to understand intercomms

Dynamic Process Creationint MPI_Comm_spawn(

char* command,char* argv[],int maxprocs,MPI_Info info,int root,MPI_Comm* intercomm,int* errorcodes[]);

Dynamic Process Creationint MPI_Comm_get_parent(

MPI_Comm* parent);

retrieves communicators parent communicator

Dynamic Process Creationint MPI_Intercomm_merge(

MPI_Comm intercomm,int high,MPI_Comm

new_intracomm)

Dynamic Process Creation…MPI_Init(&argc, &argv);makehostlist(argv[1], “targets”, &num_hosts);MPI_Info_create( &hostinfo);MPI_Info_set(hostinfo, “file”, “targets”);sprintf(soft_limit, “0:%d”, num_hosts);MPI_Info_set(hostinfo, “soft”, soft_limit);MPI_Comm_spawn(“pcp_slave”, MPI_ARGV_NULL,

num_hosts, hostinfo, 0, MPI_COMM_SELF, &pcpslaves, MPI_ERRORCODES_IGNORE);

MPI_Info_free( &hostinfo );MPI_Intercomm_merge( pcpslaves, 0, &all_procs);….

Dynamic Process Creation… // in spawned process

MPI_Init( &argc, &argv );MPI_Comm_get_parent( &slavecomm);MPI_Intercomm_merge( slavecomm, 1,&all_procs);… // now like intracomm…

Dynamic Process Creation – Multiple Executables

int MPI_Comm_spawn_multiple(int count,char* commands[],char* cmd_args[],int* maxprocs[],MPI_Info info[],int root,MPI_Comm comm,MPI_Comm * intercomm,int* errors[])

Dynamic Process Creation-Multiple executables - sample

char *array_of_commands[2] = {"ocean","atmos"}; char **array_of_argv[2]; char *argv0[] = {"-gridfile", "ocean1.grd", (char *)0}; char *argv1[] = {"atmos.grd", (char *)0}; array_of_argv[0] = argv0; array_of_argv[1] = argv1; MPI_Comm_spawn_multiple(2, array_of_commands, array_of_argv, ...);

from:http://www.epcc.ed.ac.uk/epcc-tec/document_archive/mpi-20-htm

So, What about MPI-2A lot of existing code in MPI-1MPI-1 meets a lot of scientific and engineering computing needsMPI-2 implementation not as wide spread as MPI-1MPI-2 is, at least in part, an experimental platform for research in parallel computing

MPI-2 ..for more information

http://www.mpi-forum.org/docs/