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prefix_sum.cpp
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prefix_sum.cpp
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/*
* prefix_sum.cpp
*
* Created on: Mar 23, 2012
* Author: skhajamo
*/
#include <pthread.h>
#include <iostream>
#include <math.h>
#include <time.h>
#include <sys/time.h>
using namespace std;
#define MAX_NUM 10000000
#define MAX_THREADS 4
struct thread_info_type
{
int start; // starting index of array
int end; // ending index of array
int threadId;
};
void * parallel_prefix_sum(void *);
void printArray(int arr[],int numElements);
pthread_barrier_t barr;
int inputArr[MAX_NUM]; // = {3,1,7,0,4,1,6,3};
int checkArr[MAX_NUM];
int sumArrSize = pow(2,ceil(log2(MAX_THREADS)));
int * sumArr = new int[sumArrSize];
void parallel_prefix_sum_main()
{
if(pthread_barrier_init(&barr, NULL, MAX_THREADS))
{
cout << "Could not create a barrier\n";
}
int numElements = MAX_NUM;
int numThreads = MAX_THREADS;
//cout << "Input array is " << endl;
//printArray(a,numElements);
pthread_t *p_threads= new pthread_t[numThreads];
int elementsPerThread = numElements/numThreads;
int excessElements = numElements%numThreads;
struct thread_info_type *threadInfoArr = new struct thread_info_type[numThreads];
int lastIndex = 0;
// assign ranges and create threads
for(int i=0;i<numThreads;i++)
{
threadInfoArr[i].threadId = i;
threadInfoArr[i].start = lastIndex;
lastIndex+= (elementsPerThread -1);
if(excessElements > 0)
{
lastIndex++;
excessElements--;
}
threadInfoArr[i].end = lastIndex;
pthread_create(&p_threads[i],NULL,parallel_prefix_sum,(void *)&(threadInfoArr[i]));
lastIndex++; // increment lastIndex so that it is ready for next iteration
}
for (int i=0; i< numThreads; i++)
pthread_join(p_threads[i], NULL);
//cout << "Prefix Sum array is " << endl;
//printArray(a,numElements);
}
void printArray(int arr[],int numElements)
{
for(int i=0;i<numElements;i++)
cout << arr[i] << " ";
cout << endl;
}
void * parallel_prefix_sum(void * arg)
{
struct thread_info_type threadInfo = *((struct thread_info_type *)arg);
int start = threadInfo.start; // start element Index
int end = threadInfo.end; // end element Index
int myId = threadInfo.threadId;
int numThreads = MAX_THREADS;
int numElements = end-start+1;
//cout << "Start is " << start << " End is " << end <<" id is" << myId<<endl;
// compute local prefix sums
for(int i=start+1;i<=end;i++)
inputArr[i]+=inputArr[i-1];
sumArr[myId] = inputArr[end];
//barrier to synchronize local sums
pthread_barrier_wait(&barr);
/*cout << "Sum array is ";
for(int i=0;i<sumArrSize;i++)
cout << sumArr[i] << " ";
cout << endl;
cout << "Input array is ";
for(int i=0;i<MAX_NUM;i++)
cout << inputArr[i] << " ";
cout << endl;*/
// up sweep
int numSteps = log2(sumArrSize);
int pow2 = 1,pow1;
int index,dec_i;
for(int d = 0; d < numSteps;d++)
{
pow1 = 2*pow2;
if( d == numSteps-1)
sumArr[sumArrSize-1]=0;
else
{
if(myId% pow1 ==0)
{
dec_i = myId-1;
index = dec_i+pow1;
sumArr[index] = sumArr[dec_i+pow2]+sumArr[index];
}
}
pthread_barrier_wait(&barr);
pow2=pow1;
}
/*cout << "Id is " << myId << " After Up sweep Sum array is" <<endl;
for(int i=0;i<sumArrSize;i++)
cout << sumArr[i] << " ";
cout << endl;*/
// down sweep
int index1,index2,temp;
pow1 = pow(2, numSteps);
for(int d = numSteps-1 ; d >= 0;d--)
{
pow2 = pow1/2;
if(myId% pow1 ==0)
{
dec_i = myId-1;
index2 = pow2+dec_i;
index1 = pow1+dec_i;
temp = sumArr[index2];
sumArr[index2] = sumArr[index1];
sumArr[index1] += temp;
}
pthread_barrier_wait(&barr);
pow1 = pow2;
}
/*
cout << "Id is " << myId << "After Down sweep Sum array is" <<endl;
for(int i=0;i<sumArrSize;i++)
cout << sumArr[i] << " ";
cout << endl;*/
//compute complete prefix sum
for(int i=start;i<=end;i++)
inputArr[i]+= sumArr[myId];
}
int main()
{
//cout << "Sum Arr size is "<< sumArrSize << endl;
for(int i=0;i<MAX_NUM;i++)
{
inputArr[i] = i+1;
checkArr[i] = inputArr[i];
}
struct timeval tz;
struct timezone tx;
double start_time, end_time;
gettimeofday(&tz, &tx);
start_time = (double)tz.tv_sec + (double) tz.tv_usec / 1000000.0;
parallel_prefix_sum_main();
gettimeofday(&tz, &tx);
end_time = (double)tz.tv_sec + (double) tz.tv_usec / 1000000.0;
double ptime =(double)end_time - (double)start_time;
cout << "Parallel Time is " << ptime;
gettimeofday(&tz, &tx);
start_time = (double)tz.tv_sec + (double) tz.tv_usec / 1000000.0;
for(int i=1;i<MAX_NUM;i++)
checkArr[i] += checkArr[i-1];
gettimeofday(&tz, &tx);
end_time = (double)tz.tv_sec + (double) tz.tv_usec / 1000000.0;
double stime = ((double)end_time - (double)start_time);
cout << "Serial Time is " << stime;
cout << "Speedup is "<< stime/ptime <<endl;
for(int i=0;i<MAX_NUM;i++)
{
if(inputArr[i]!=checkArr[i])
cout << "Error " << "i is "<<i<< " parallel result is "<<inputArr[i] << "serial result is " <<checkArr[i]<<endl;
}
}