我正在学习关于莱斯大学提供的并行编程的课程。在赋值中，我们应该递归地计算数组元素的倒数之和。为了并行化，我们使用了一个fork / join池，并递归地计算了左右子阵列的所需总和并将它们连接起来。但是我得到的执行时间比我按顺序执行时更多。以下是eclipse中的控制台输出 -
顺序时间：0.136，值：7.485471 平行时间：6.674，价值：7.485471
代码如下 -
package edu.coursera.parallel;
import java.util.Random;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
/**
* Class wrapping methods for implementing reciprocal array sum in parallel.
*/
public final class ReciprocalArraySum
{
/**
* Default constructor.
*/
private ReciprocalArraySum()
{
}
/**
* Sequentially compute the sum of the reciprocal values for a given array.
*
* @param input Input array
* @return The sum of the reciprocals of the array input
*/
protected static double seqArraySum(final double[] input)
{
double sum = 0;
// Compute sum of reciprocals of array elements
for (int i = 0; i < input.length; i++)
{
sum += (1 / input[i]);
}
return sum;
}
/**
* Computes the size of each chunk, given the number of chunks to create
* across a given number of elements.
*
* @param nChunks The number of chunks to create
* @param nElements The number of elements to chunk across
* @return The default chunk size
*/
private static int getChunkSize(final int nChunks, final int nElements)
{
// Integer ceil
return (nElements + nChunks - 1) / nChunks;
}
/**
* Computes the inclusive element index that the provided chunk starts at,
* given there are a certain number of chunks.
*
* @param chunk The chunk to compute the start of
* @param nChunks The number of chunks created
* @param nElements The number of elements to chunk across
* @return The inclusive index that this chunk starts at in the set of
* nElements
*/
private static int getChunkStartInclusive(final int chunk, final int nChunks, final int nElements)
{
final int chunkSize = getChunkSize(nChunks, nElements);
return chunk * chunkSize;
}
/**
* Computes the exclusive element index that the provided chunk ends at,
* given there are a certain number of chunks.
*
* @param chunk The chunk to compute the end of
* @param nChunks The number of chunks created
* @param nElements The number of elements to chunk across
* @return The exclusive end index for this chunk
*/
private static int getChunkEndExclusive(final int chunk, final int nChunks, final int nElements)
{
final int chunkSize = getChunkSize(nChunks, nElements);
final int end = (chunk + 1) * chunkSize;
if (end > nElements)
{
return nElements;
}
else
{
return end;
}
}
/**
* This class stub can be filled in to implement the body of each task
* created to perform reciprocal array sum in parallel.
*/
private static class ReciprocalArraySumTask extends RecursiveAction
{
/**
* Starting index for traversal done by this task.
*/
private final int startIndexInclusive;
/**
* Ending index for traversal done by this task.
*/
private final int endIndexExclusive;
/**
* Input array to reciprocal sum.
*/
private final double[] input;
/**
* Intermediate value produced by this task.
*/
private double value;
/**
* Constructor.
* @param setStartIndexInclusive Set the starting index to begin
* parallel traversal at.
* @param setEndIndexExclusive Set ending index for parallel traversal.
* @param setInput Input values
*/
ReciprocalArraySumTask(final int setStartIndexInclusive, final int setEndIndexExclusive, final double[] setInput)
{
this.startIndexInclusive = setStartIndexInclusive;
this.endIndexExclusive = setEndIndexExclusive;
this.input = setInput;
}
/**
* Getter for the value produced by this task.
* @return Value produced by this task
*/
public double getValue()
{
return value;
}
@Override
protected void compute()
{
// TODO
if((endIndexExclusive - startIndexInclusive) <= (input.length/5))
{
for(int i=startIndexInclusive;i<endIndexExclusive;i++)
{
value+=(1/input[i]);
}
}
else
{
int mid = ((startIndexInclusive+endIndexExclusive)/2);
ReciprocalArraySumTask l = new ReciprocalArraySumTask(startIndexInclusive, mid, input);
ReciprocalArraySumTask r = new ReciprocalArraySumTask(mid, endIndexExclusive, input);
l.fork();
r.fork();
l.join();
r.join();
value = l.getValue()+r.getValue();
}
}
}
/**
* TODO: Modify this method to compute the same reciprocal sum as
* seqArraySum, but use two tasks running in parallel under the Java Fork
* Join framework. You may assume that the length of the input array is
* evenly divisible by 2.
*
* @param input Input array
* @return The sum of the reciprocals of the array input
*/
protected static double parArraySum(final double[] input)
{
assert input.length % 2 == 0;
double sum = 0.0;
// Compute sum of reciprocals of array elements
/*for(int i=0;i<input.length;i++)
{
sum+=(1/input[i]);
}*/
ReciprocalArraySumTask t = new ReciprocalArraySumTask(0,input.length,input);
t.compute();
sum = t.getValue();
return sum;
}
/**
* TODO: Extend the work you did to implement parArraySum to use a set
* number of tasks to compute the reciprocal array sum. You may find the
* above utilities getChunkStartInclusive and getChunkEndExclusive helpful
* in computing the range of element indices that belong to each chunk.
*
* @param input Input array
* @param numTasks The number of tasks to create
* @return The sum of the reciprocals of the array input
*/
protected static double parManyTaskArraySum(final double[] input, final int numTasks)
{
double sum = 0;
// Compute sum of reciprocals of array elements
for (int i = 0; i < input.length; i++)
{
sum += 1 / input[i];
}
return sum;
}
public static void main(String[] args)
{
ReciprocalArraySum r = new ReciprocalArraySum();
double[] imp = new double[1000];
Random rGen = new Random();
for(int i=0;i<1000;i++)
{
imp[i] = (i+1);
}
long startTimeSeq = System.nanoTime();
double value = r.seqArraySum(imp);
long endTimeSeq = System.nanoTime();
System.out.printf("Time for sequential: %4.3f , Value: %f\n",(endTimeSeq-startTimeSeq)/1e6,value);
long startTimePar = System.nanoTime();
value = r.parArraySum(imp);
long endTimePar = System.nanoTime();
System.out.printf("Time for parallel: %4.3f , Value: %f",(endTimePar-startTimePar)/1e6,value);
}
}
答案 0 :(得分：1)
您为每个join()使用fork()。 join() stalls the program甚至使用Java 8.尝试使用CountedCompleter类。它是散射 - 聚集的一种形式，不会失速。它取代了流中使用的join()。