Parallel Programming with PVM Prof. Sivarama Dandamudi School of Computer Science Carleton University

© S. Dandamudi2 Parallel Algorithm Models  Five basic models  Data parallel model  Task graph model  Work pool model  Master-slave model  Pipeline model  Hybrid models

Carleton University© S. Dandamudi3 Parallel Algorithm Models (cont’d)  Data parallel model  One of the simplest of all the models  Tasks are statically mapped onto processors  Each task performs similar operation on different data  Called data parallelism model  Work may be done in phases  Operations in different phases may be different  Ex: Matrix multiplication

Carleton University© S. Dandamudi4 Parallel Algorithm Models (cont’d)  Data parallel model A11 A12 B11 B12 C11 C12 A21 A22 B21 B22 C21 C22  A decomposition into four tasks  Task 1: C11 = A11 B11 + A12 B21  Task 2: C12 = A11 B12 + A12 B22  Task 3: C21 = A21 B11 + A22 B21  Task 4: C11 = A21 B21 + A22 B22. =

Carleton University© S. Dandamudi5 Parallel Algorithm Models (cont’d)  Task graph model  Parallel algorithm is viewed as a task-dependency graph  Called task parallelism model  Typically used for tasks that have large amount of data  Static mapping is used to optimize data movement cost  Locality-based mapping is important  Ex: Divide-and-conquer algorithms, parallel quicksort

Carleton University© S. Dandamudi6 Parallel Algorithm Models (cont’d)  Task parallelism

Carleton University© S. Dandamudi7 Parallel Algorithm Models (cont’d)  Work pool model  Dynamic mapping of tasks onto processors  Important for load balancing  Used on message passing systems  When the data associated with a task is relatively small  Granularity of tasks  Too small: overhead in accessing tasks can increase  Too big: Load imbalance  Ex: Parallelization of loops by chunk scheduling

Carleton University© S. Dandamudi8 Parallel Algorithm Models (cont’d)  Master-slave model  One or more master processes generate work and allocate it to worker processes  Also called manager-worker model  Suitable for both shared-memory and message passing systems  Master can potentially become a bottleneck  Granularity of tasks is important

Carleton University© S. Dandamudi9 Parallel Algorithm Models (cont’d)  Pipeline model  A stream of data passes through a series of processors  Each process performs some task on the data  Also called stream parallelism model  Uses producer-consumer relationship  Overlapped execution  Useful in applications such as database query processing  Potential problem  One process can delay the whole pipeline

Carleton University© S. Dandamudi10 Parallel Algorithm Models (cont’d)  Pipeline model R5 R4 R3 R2R1 Pipelined processing can avoid writing temporary results on disk and reading them back

Carleton University© S. Dandamudi11 Parallel Algorithm Models (cont’d)  Hybrid models  Possible to use multiple models  Hierarchical  Different models at different levels  Sequentially  Different models in different phases  Ex: Major computation may use task graph model  Each node of the graph may use data parallelism or pipeline model

Carleton University© S. Dandamudi12 PVM  Parallel virtual machine  Collaborative effort  Oak Ridge National Lab, University of Tennessee, Emory University, and Carnegie Mellon University  Began in 1989  Version 1.0 was used internally  Version 2.0 released in March 1991  Version 3.0 in February 1993

Carleton University© S. Dandamudi13 PVM (cont’d)  Parallel virtual machine  Targeted for heterogeneous network computing  Different architectures  Data formats  Computational speeds  Machine loads  Network load

Carleton University© S. Dandamudi14 PVM Calls  Process control int tid = pvm_mytid(void)  Returns pid of the calling process  Can be called multiple times int info = pvm_exit(void)  Does not kill the process  Tells the local pvmd that this process is leaving PVM  info < 0 indicates error (error: pvmd not responding)

Carleton University© S. Dandamudi15 PVM Calls (cont’d)  Process control int numt = pvm_spawn(char *task, char **argv, int flag, char *where, int ntask, int *tids)  Starts ntask copies of the executable file task Arguments to task (NULL terminated) Specific host Specific architecture (PVM_ARCH)

Carleton University© S. Dandamudi16 PVM Calls (cont’d)  flag specifies options ValueOptionMeaning 0 PvmTaskDefault PVM chooses where to span 1 PvmTaskHost where specifies a host 2 PvmTaskArch where specifies a architecture 3 PvmTaskDebug Starts tasks under debugger

Carleton University© S. Dandamudi17 PVM Calls (cont’d)  Process control int info = pvm_kill(int tid)  Kills the PVM task identified by tid  Does not kill the calling task  To kill the calling task  First call pvm_exit()  Then exit()  Writes to the file /tmp/pvml.

Carleton University© S. Dandamudi18 PVM Calls (cont’d)  Information int tid = pvm_parent(void)  Returns the tid of the process that spawned the calling task  Returns PvmNoParent if the task is not created by pvm_spawn()

Carleton University© S. Dandamudi19 PVM Calls (cont’d)  Information int info = pvm_config( int *nhost, int *narch, struct pvmhostinfo **hostp)  Returns nhost = number of hosts  Returns narch = number of different data formats

Carleton University© S. Dandamudi20 PVM Calls (cont’d)  Message sending  Involves three steps  Send buffer must be initialized  Use pvm_initsend()  Message must be packed  Use pvm_pk*()  Several pack routines are available  Send the message  Use pvm_send()

Carleton University© S. Dandamudi21 PVM Calls (cont’d)  Message sending int bufid = pvm_initsend( int encoding)  Called before packing a new message into the buffer  Clears the send buffer and creates a new one for packing a new message  bufid = new buffer id

Carleton University© S. Dandamudi22 PVM Calls (cont’d)  encoding can have three options:  PvmDataDefault  XDR encoding is used by default  Useful for heterogeneous architectures  PvmDataRaw  No encoding is done  Messages sent in their original form  PvmDataInPLace  No buffer copying  Buffer should not be modified until sent

Carleton University© S. Dandamudi23 PVM Calls (cont’d)  Packing data  Several routines are available (one for each data type)  Each takes three arguments int info = pvm_pkbyte(char *cp, int nitem, int stride)  nitem = # items to be packed  stride = stride in elements

Carleton University© S. Dandamudi24 PVM Calls (cont’d)  Packing data pvm_pkint pvm_pklong pvm_pkfloat pvm_pkdouble pvm_pkshort  Pack string routine requires only the NULL-terminated string pointer pvm_pkstr(char *cp)

Carleton University© S. Dandamudi25 PVM Calls (cont’d)  Sending data int info = pvm_send(int tid, int msgtag)  Sends the message in the packed buffer to task tid  Message is tagged with msgtag  Message tags are useful to distinguish different types of messages

Carleton University© S. Dandamudi26 PVM Calls (cont’d)  Sending data (multicast) int info = pvm_mcast(int *tids, int ntask, int msgtag)  Sends the message in the packed buffer to all tasks in the tid array (except itself)  tid array length is given by ntask

Carleton University© S. Dandamudi27 PVM Calls (cont’d)  Receiving data  Two steps  Receive data  Unpack it  Two versions  Blocking  Waits until the message arrives  Non-blocking  Does not wait

Carleton University© S. Dandamudi28 PVM Calls (cont’d)  Receiving data  Blocking receive int info = pvm_recv(int tid, int msgtag)  Wait until a message with msgtag has arrived from task tid  Wildcard value (  1) is allowed for both msgtag and tid

Carleton University© S. Dandamudi29 PVM Calls (cont’d)  Receiving data  Non-blocking receive int info = pvm_nrecv(int tid, int msgtag)  If no message with msgtag has arrived from task tid  Returns bufid = 0  Otherwise, behaves like the blocking receive

Carleton University© S. Dandamudi30 PVM Calls (cont’d)  Receiving data  Probing for a message int info = pvm_probe(int tid, int msgtag)  If no message with msgtag has arrived from task tid  Returns bufid = 0  Otherwise, returns a bufid for the message  Does not receive the message

Carleton University© S. Dandamudi31 PVM Calls (cont’d)  Unpacking data (similar to packing routines) pvm_upkint pvm_upklong pvm_upkfloat pvm_upkdouble pvm_upkshort pvm_upkbyte  Pack string routine requires only the NULL-terminated string pointer pvm_upkstr(char *cp)

Carleton University© S. Dandamudi32 PVM Calls (cont’d)  Buffer information  Useful to find the size of the received message int info = pvm_bufinfo(int bufid, int *bytes, int *msgtag, int *tid)  Returns msgtag, source tid, and size in bytes

Carleton University© S. Dandamudi33 Example  Finds sum of elements of a given vector  Vector size is given as input  The program can be run on a PVM with up to 10 nodes  Can be modified by changing a constant  Vector is assumes to be evenly divisable by number of nodes in PVM  Easy to modify this restriction  Master ( vecsum.c ) and slave ( vecsum_slave.c ) programs

Carleton University© S. Dandamudi34 Example (cont’d) vecsum.c #include #include "pvm3.h" #define MAX_SIZE /* max. vector size */ #define NPROCS 10 /* max. number of PVM nodes */

Carleton University© S. Dandamudi35 Example (cont’d) main() { int cc, tid[NPROCS]; long vector[MAX_SIZE]; double sum = 0, partial_sum; /* partial sum received from slaves */ long i, vector_size;

Carleton University© S. Dandamudi36 Example (cont’d) int nhost, /* actual # of hosts in PVM */ size; /* size of vector to be distributed */ struct timeval start_time, finish_time; long sum_time;

Carleton University© S. Dandamudi37 Example (cont’d) printf("Vector size = "); scanf("%ld", &vector_size); for(i=0; i<vector_size; i++) /* initialize vector */ vector[i] = i; gettimeofday(&start_time, (struct timezone*)0); /* start time */

Carleton University© S. Dandamudi38 Example (cont’d) tid[0] = pvm_mytid(); /* establish my tid */ /* get # of hosts using pvm_config() */ pvm_config(&nhost, (int *)0, (struct hostinfo *)0); size = vector_size/nhost; /* size of vector to send to slaves */

Carleton University© S. Dandamudi39 Example (cont’d) if (nhost > 1) pvm_spawn("vecsum_slave", (char **)0, 0, "", nhost-1, &tid[1]); for (i=1; i<nhost;i++){ /* distribute data to slaves */ pvm_initsend(PvmDataDefault); pvm_pklong(&vector[i*size],size,1); pvm_send(tid[i],1); }

Carleton University© S. Dandamudi40 Example (cont’d) for (i=0; i<size;i++) /* perform local sum */ sum += vector[i]; for (i=1; i<nhost;i++){ /* collect partial sums from slaves */ pvm_recv(-1,2); pvm_upkdouble(&partial_sum,1,1); sum += partial_sum; }

Carleton University© S. Dandamudi41 Example (cont’d) gettimeofday(&finish_time, (struct timezone*)0); /* finish time */ sum_time = (finish_time.tv_sec – start_time.tv_sec) * finish_time.tv_usec - start_time.tv_usec; Time in  secs

Carleton University© S. Dandamudi42 Example (cont’d) printf("Sum = %lf\n",sum); printf("Sum time on %d hosts = %lf sec\n", nhost, (double)sum_time/ ); pvm_exit(); }

Carleton University© S. Dandamudi43 Example (cont’d) vecsum_slave.c #include "pvm3.h" #define MAX_SIZE main() { int ptid, bufid, vector_bytes; long vector[MAX_SIZE]; double sum = 0; int i;

Carleton University© S. Dandamudi44 Example (cont’d) ptid = pvm_parent(); /* find parent tid */ bufid = pvm_recv(ptid,1); /* receive data from master */ /* use pvm_bufinfo() to find the number of bytes received */ pvm_bufinfo(bufid, &vector_bytes, (int *)0, (int *) 0);

Carleton University© S. Dandamudi45 Example (cont’d) pvm_upklong(vector, vector_bytes/sizeof(long),1); /* unpack */ for (i=0; i<vector_bytes/sizeof(long); i++) /* local summation */ sum += vector[i];

Carleton University© S. Dandamudi46 Example (cont’d) pvm_initsend(PvmDataDefault); /* send sum to master */ pvm_pkdouble(&sum,1,1); pvm_send(ptid, 2); /* use msg type 2 for partial sum */ pvm_exit(); }