WSE 187: INTRODUCTION TO PARALLEL PROGRAMMING* Lecture 2 Jesmin Jahan Tithi *Prepared with the help of free online resources.

LOGIN TO SSH Steps (Windows)  Connect to the host  Give provided password when prompted For the first time users:  Accept security keys  Change password:  First provide the Old password  Then type new password  Repeat new password Do not afraid if you do not see any character On screen. But be careful when you are Typing.  Save the host info in SSH when prompted.  Mac uses:  Use terminal to directly connect to the server.  Other steps are the same.  You may try to use Filezilla to transfer file from mac to server.

HELLO PARALLEL WORLD! Intel Cilk Plus

CILK PLUS Intel® Cilk™ Plus = add-on to the C and C++, implemented by the Intel® C++ Compiler 3 keywords to C and C++: cilk_for, cilk_spawn, and cilk_sync cilk_spawn - Specifies that a function call can execute asynchronously, without requiring the caller to wait for it to return. This is an expression of an opportunity for parallelism, not a command that mandates parallelism. The Intel Cilk Plus runtime will choose whether to run the function in parallel with its caller.cilk_spawn cilk_sync - Specifies that all spawned calls in a function must complete before execution continues. There is an implied cilk_sync at the end of every function that contains a cilk_spawn.cilk_sync cilk_for - Allows iterations of the loop body to be executed in parallel.cilk_for cilk_spawn and cilk_for keywords express opportunities for parallelism.

CILK_SPAWN #include static void hello(){ int i=0; for(i=0;i< ;i++) printf(""); printf("Hello "); } static void world(){ int i=0; for(i=0;i< ;i++) printf(""); printf("world! "); } int main(){ cilk_spawn hello(); cilk_spawn world(); //cilk_sync; printf("Done! "); } Compile: icc –O3 –o hello Hello_parallel_world.cpp Run:./hello Run: CILK_NWORKERS=4./hello

CILK_SPAWN EXERCISE Order of placement Wheels, Chassis, Engine, Frame, Steering wheel #include void make(char* str){ int i=0; for(i=0;i< ;i++) printf(""); printf("%s has/have been created.\n",str); } void place(char* str){ int i=0; for(i=0;i< ;i++) printf(""); printf("%s has/have been placed.\n",str); } int main(){ //Place your code here }

CILK_FOR #include #include "cilktime.h" using namespace std; #define n int main(){ // First input vector. int A[n]; // Second input vector. int B[n]; // Sum vector. int C[n]; // Initialize the vectors or arrays with input. cilk_for (int i = 0; i <= n; i++){ A[i] = i; B[i] = i+1; } // Compute the sum unsigned long long tstart = cilk_getticks(); //beginning time stamp cilk_for (int i = 0; i <= n; i++){ C[i] = A[i] + B[i]; } unsigned long long tend = cilk_getticks(); //end time stamp cout<<"Time to run:"<<cilk_ticks_to_seconds(tend-tstart)<<endl; // Check the sum to verify. int pos; cout<<"Enter position of element to inspect"<<endl; cin>>pos; cout<<C[pos]<<endl; return 0; } for (int i = 0; i < 8; ++i) { cilk_spawn do_work(i); } cilk_sync; A better approach is to use a cilk_for loop: cilk_for (int i = 0; i < 8; ++i) { do_work(i); }

CILK_FOR #include int main(){ long int sum = 0; cilk_for (int i = 0; i <= ; i++) sum += i; printf("%ld\n",sum); return 0; } for (int i = 0; i < 8; ++i) { cilk_spawn do_work(i); } cilk_sync; A better approach is to use a cilk_for loop: cilk_for (int i = 0; i < 8; ++i) { do_work(i); } //wrong! race conditionrace condition

CILK_FOR #include #include //pthread library int main(){ long int sum = 0; pthread_mutex_t m; //define the lock pthread_mutex_init(&m,NULL); //initialize the lock cilk_for (int i = 0; i <= ; i++){ pthread_mutex_lock(&m); //lock - prevents other threads from running this code sum += i; pthread_mutex_unlock(&m); //unlock - allows other threads to access this code } printf("%ld\n",sum); } Several ways of dealing with race conditions. First option: Use locks! We will learn more later.

CHANGING NUMBER OF CORES/THREADS Run with: CILK_NWORKERS=4./executable Or change inside the main program: if (0!= __cilkrts_set_param("nworkers","16")) { cout<<"Failed to set worker count\n"<<endl; return 1; } Check to verify: int num_threads =__cilkrts_get_nworkers(); cout<< num_threads <<endl;