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// 2D FFT Using threads // George F. Riley, Georgia Tech, Fall 2009 #include <iostream> #include "pthread.h" #include "math.h" #include <sys/time.h> #include "complex.h" #include "InputImage.h" using namespace std; // We use global variables in lieu of member variables for this example Complex** h; // Points to the 2D array of complex (the input) Complex* W; // Weights (computed once in main unsigned N; // Number of elements (both width and height) unsigned nThreads; // Desired number of threads unsigned activeCount = 0; // Number of active threads // pthread variables pthread_mutex_t activeMutex; pthread_mutex_t exitMutex; pthread_cond_t exitCondition; pthread_mutex_t coutMutex; // Add a verbose flag to turn on/off extra outputs bool verbose = false; // Helper routines void DumpTransformedValues() { // Code omitted for brevity } void TransposeInPlace(Complex** m, int wh) { // code omitted for brevity }
void LoadWeights() { // Compute the needed W values. }
Omitted for brevity
void Transform1D(Complex* h) { // The simple 1D transform we did earlier. }
Code omitted for brevity
Program threaded-fft.cc
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void* FFT_Thread(void* v) { unsigned long myId = (unsigned long)v; // My thread number unsigned rowsPerCPU = N / nThreads; unsigned myFirstRow = myId * rowsPerCPU; // We have to do a mutex around the "activeCount++". Why? pthread_mutex_lock(&activeMutex); activeCount++; pthread_mutex_unlock(&activeMutex); if (verbose) { pthread_mutex_lock(&coutMutex); cout << "MyId is " << myId << " myFirstRow " << myFirstRow << endl; pthread_mutex_unlock(&coutMutex); } // Call the 1D FFT on each row for (unsigned i = 0; i < rowsPerCPU; ++i) { Transform1D(h[myFirstRow + i]); } // Now notify the main thread we have completed the rows pthread_mutex_lock(&exitMutex); // Insure only one thread signals the exit pthread_mutex_lock(&activeMutex); // Insure only one thread changes active activeCount--; pthread_mutex_unlock(&activeMutex); (activeCount if == 0) { // We are the last thread to exit. Signal the main thread // that all threads are done pthread_cond_signal(&exitCondition); } pthread_mutex_unlock(&exitMutex); return 0; }
Program threaded-fft.cc (continued)
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int main( int argc, char** argv) { verbose = argc > 3; InputImage image(argv[1]); nThreads = atol(argv[2]); // Number of threads N = image.GetHeight(); // Assume square, width = height h = image.GetRows(0, N); // In this case, we get all rows // Start the timer here, after loading the image struct timeval tp; gettimeofday(&tp, 0); double startSec = tp.tv_sec + tp.tv_usec/1000000.0; LoadWeights(); // Only need to do this once
// Initialize the pthread mutex and condition variables pthread_mutex_init(&activeMutex, 0); pthread_mutex_init(&exitMutex, 0); pthread_cond_init(&exitCondition, 0); pthread_mutex_init(&coutMutex, 0); // We lock the exitMutex to be sure no threads exit until // all threads created, and we are waiting on the condition signal pthread_mutex_lock(&exitMutex); // Create the threads for (unsigned i = 0; i < nThreads; ++i) { pthread_t t; pthread_create(&t, 0, FFT_Thread, (void*)i); } // Now wait for them to finish pass 1 pthread_cond_wait(&exitCondition, &exitMutex); if (verbose) cout << "All threads finished pass 1" << endl; // Transpose the matrix and schedule threads to do rows again TransposeInPlace(h, N); // Start the threads again for (unsigned i = 0; i < nThreads; ++i) { pthread_t t; pthread_create(&t, 0, FFT_Thread, (void*)i); } // Now wait for them to finish pass 2 pthread_cond_wait(&exitCondition, &exitMutex); if (verbose) cout << "All threads finished pass 2" << endl; // Transpose back and write results TransposeInPlace(h, N); gettimeofday(&tp, 0); cout << "Calculated FFT " << (tp.tv_sec+tp.tv_usec/1000000.0) - startSec << " seconds" << endl; DumpTransformedValues(); }
Program threaded-fft.cc (continued)
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