Java 多线程快速排序比预期慢得多
基本上,我已经花了几个小时研究实现递归和多线程快速排序和合并排序的最佳方法(这篇文章只是关于快速排序)。我的目标是将给定计算机中的每个逻辑处理器都固定在一起,以获得最大的快速排序速度 我采取的方法是在创建线程时递归地划分问题,直到数组被排序或者我达到cpu上的处理器数量,在这种情况下,问题的其余部分不会被划分到新线程上,而是在它们自己的核心上执行 在创建了一个只能在我的计算机上工作的非常基本的解决方案之后,我遇到了我试图在下面使用的Fork/Join框架,但我真的不知道如何使用它。我想到的是,在排序10000000个从0到1000的随机数时比单线程的排序慢,但我仍然认为它很有趣,因为它说,它能够从较慢的线程中窃取工作,不管这意味着什么 然后,我最近听说了线程池,最初创建所有线程并分发它们,因为创建新线程对系统来说很费力。但我从未尝试过实现这一点。也许我对Fork/Join的理解有偏差,我想知道是否有人能给我指出正确的方向,或者告诉我在我当前的程序中我做错了什么 下面你会发现我在多线程快速排序和单线程快速排序方面的尝试,这正是我试图转换为多线程排序的原因。感谢您的帮助。干杯Java 多线程快速排序比预期慢得多,java,multithreading,quicksort,Java,Multithreading,Quicksort,基本上,我已经花了几个小时研究实现递归和多线程快速排序和合并排序的最佳方法(这篇文章只是关于快速排序)。我的目标是将给定计算机中的每个逻辑处理器都固定在一起,以获得最大的快速排序速度 我采取的方法是在创建线程时递归地划分问题,直到数组被排序或者我达到cpu上的处理器数量,在这种情况下,问题的其余部分不会被划分到新线程上,而是在它们自己的核心上执行 在创建了一个只能在我的计算机上工作的非常基本的解决方案之后,我遇到了我试图在下面使用的Fork/Join框架,但我真的不知道如何使用它。我想到的是,在
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
public class MultithreadedQuicksort {
public static void main(String[] args) {
List<Comparable> nums = new ArrayList<Comparable>();
Random rand = new Random();
for (int i=0; i<10000000; i++) {
nums.add(rand.nextInt(1000));
}
long start = System.currentTimeMillis();
Quicksort quickSort = new Quicksort(nums, 0, nums.size() -1);
ForkJoinPool pool = new ForkJoinPool();
pool.invoke(quickSort);
long end = System.currentTimeMillis();
System.out.println(end - start);
System.out.println(nums.size());
}
}
class Quicksort extends RecursiveAction {
int first;
int last;
private List<Comparable> nums;
Comparable midValue;
int midIndex;
int low;
int high;
public Quicksort(List<Comparable> nums){
this.nums=nums;
this.low = 0;
this.high = nums.size() - 1;
}
public Quicksort(List<Comparable> nums, int first, int last) {
this.first = first;
this.last = last;
this.nums = nums;
this.low = first;
this.high = last;
this.midIndex = (first + last) / 2;
this.midValue = nums.get(midIndex);
}
@Override
protected void compute() {
split();
if (high > first)
invokeAll(new Quicksort(nums, first, high));
if (low < last)
invokeAll(new Quicksort(nums, low, last));
}
public void split() {
while(low < high) {
while (nums.get(low).compareTo(midValue) < 0) {
low++;
}
while (nums.get(high).compareTo(midValue) > 0) {
high--;
}
if (low <= high) {
swap(low, high);
low++;
high--;
}
}
}
public void swap(int index1, int index2)
{
Comparable temp;
temp = nums.get(index1);
nums.set(index1, nums.get(index2));
nums.set(index2, temp);
}
}
import java.util.ArrayList;
导入java.util.List;
导入java.util.Random;
导入java.util.concurrent.ForkJoinPool;
导入java.util.concurrent.RecursiveAction;
公共类多线程快速排序{
公共静态void main(字符串[]args){
List nums=new ArrayList();
Random rand=新的Random();
for(int i=0;i优先)
invokeAll(新快速排序(nums,first,high));
如果(低<上次)
invokeAll(新的快速排序(nums、low、last));
}
公开作废拆分(){
while(低<高){
while(nums.get(低).compareTo(中值)<0){
低++;
}
而(nums.get(高).compareTo(中值)>0){
高--;
}
如果(低0){
高--;
}
如果(先低)
快速排序(nums,第一,高);
如果(低<上次)
快速排序(nums、low、last);
}
我对java不太了解,因此下面的示例代码对于多线程来说可能是runnable的笨拙用法。此示例代码使用8个线程,qsortmt()执行一个分区并启动qsort0()的两个实例。qsort0()的每个实例执行一个分区并调用qsort1()的两个实例。qsort1()的每个实例执行分区并调用qsort2()的两个实例。qsort2()的每个实例都调用qsort()。对于本例中使用的1600万个整数,8线程排序大约需要1秒,而非线程排序大约需要1.6秒,因此节省的时间不多。部分问题是分区步骤是在调用线程对子分区进行操作之前完成的
切换到C++和Windows本地线程,8个线程花费了大约0.632秒,无线程大约1.352秒。切换到合并排序,将数组拆分为8个部分,对每个部分进行排序,然后合并8个部分大约0.40秒,单线程大约1.45秒。
package x;
import java.util.Random;
public class x {
class qsort0 implements Runnable
{
int[] a;
int lo;
int hi;
private qsort0(int[] a, int lo, int hi)
{
this.a = a;
this.lo = lo;
this.hi = hi;
}
@Override
public void run()
{
if(this.lo >= this.hi)
return;
int pi = partition(this.a, this.lo, this.hi);
Thread lt = new Thread(new qsort1(a, this.lo, pi));
Thread rt = new Thread(new qsort1(a, pi+1, this.hi));
lt.start();
rt.start();
try {lt.join();} catch (InterruptedException ex){}
try {rt.join();} catch (InterruptedException ex){}
}
}
class qsort1 implements Runnable
{
int[] a;
int lo;
int hi;
private qsort1(int[] a, int lo, int hi)
{
this.a = a;
this.lo = lo;
this.hi = hi;
}
@Override
public void run()
{
if(this.lo >= this.hi)
return;
int pi = partition(this.a, this.lo, this.hi);
Thread lt = new Thread(new qsort2(a, this.lo, pi));
Thread rt = new Thread(new qsort2(a, pi+1, this.hi));
lt.start();
rt.start();
try {lt.join();} catch (InterruptedException ex){}
try {rt.join();} catch (InterruptedException ex){}
}
}
class qsort2 implements Runnable
{
int[] a;
int lo;
int hi;
private qsort2(int[] a, int lo, int hi)
{
this.a = a;
this.lo = lo;
this.hi = hi;
}
@Override
public void run() {
if(this.lo >= this.hi)
return;
qsort(this.a, this.lo, this.hi);
}
}
// quicksort multi-threaded
@SuppressWarnings("empty-statement")
public static void qsortmt(int[] a, int lo, int hi)
{
if(lo >= hi)
return;
int pi = partition(a, lo, hi);
Thread lt = new Thread(new x().new qsort0(a, lo, pi));
Thread rt = new Thread(new x().new qsort0(a, pi+1, hi));
lt.start();
rt.start();
try {lt.join();} catch (InterruptedException ex){}
try {rt.join();} catch (InterruptedException ex){}
}
@SuppressWarnings("empty-statement")
public static int partition(int []a, int lo, int hi)
{
int md = lo+(hi-lo)/2;
int ll = lo-1;
int hh = hi+1;
int t;
int p = a[md];
while(true){
while(a[++ll] < p);
while(a[--hh] > p);
if(ll >= hh)
break;
t = a[ll];
a[ll] = a[hh];
a[hh] = t;
}
return hh;
}
@SuppressWarnings("empty-statement")
public static void qsort(int[] a, int lo, int hi)
{
while(lo < hi){
int ll = partition(a, lo, hi);
int hh = ll+1;
// recurse on smaller part, loop on larger part
if((ll - lo) <= (hi - hh)){
qsort(a, lo, ll);
lo = hh;
} else {
qsort(a, hh, hi);
hi = ll;
}
}
}
public static void main(String[] args)
{
int[] a = new int[16*1024*1024];
Random r = new Random(0);
for(int i = 0; i < a.length; i++)
a[i] = r.nextInt();
long bgn, end;
bgn = System.currentTimeMillis();
qsortmt(a, 0, a.length-1);
end = System.currentTimeMillis();
for(int i = 1; i < a.length; i++){
if(a[i-1] > a[i]){
System.out.println("failed");
break;
}
}
System.out.println("milliseconds " + (end-bgn));
}
}
x包;
导入java.util.Random;
公共x类{
类qsort0实现可运行
{
int[]a;
内卢;
int hi;
私人qsort0(整数[]a,整数低,整数高)
{
这个a=a;
this.lo=lo;
hi=hi;
}
@凌驾
公开募捐
{
如果(this.lo>=this.hi)
返回;
int pi=分区(this.a,this.lo,this.hi);
线程lt=新线程(新的qsort1(a,this.lo,pi));
Thread rt=新线程(新的qsort1(a,pi+1,this.hi));
lt.start();
rt.start();
尝试{lt.join();}catch(InterruptedException ex){}
尝试{rt.join();}catch(InterruptedException ex){}
}
}
类qsort1实现可运行
{
int[]a;
内卢;
int hi;
私人qsort1(整数[]a,整数低,整数高)
{
这个a=a;
this.lo=lo;
hi=hi;
}
@凌驾
公开募捐
{
如果(this.lo>=this.hi)
返回;
int pi=分区(this.a,this.lo,this.hi);
线程lt=新线程(新的qsort2(a,this.lo,pi));
Thread rt=新线程(新的qsort2(a,pi+1,this.hi));
lt.start();
rt.start();
尝试{lt.join();}catch(InterruptedException ex){}
尝试{rt.join();}catch(InterruptedException ex){}
}
}
类qsort2实现可运行
{
int[]a;
内卢;
int hi;
私人qsort2(整数[]a,整数低,整数高)
{
这个a=a;
this.lo=lo;
hi=hi;
}
@凌驾
公开募捐{
如果(this.lo>=this.hi)
返回;
qsort(this.a,this.lo,this.hi);
}
}
//多线程快速排序
@SuppressWarnings(“空语句”)
公共静态无效qsortmt(整数[]a,整数低,整数高)
{
如果(低>=高)
返回;
int pi=分区(a,lo,hi);
线程lt=新线程(新x().新qsort0(a,lo,pi));
线程rt=新线程(新x().新qsort0(a,pi+1,hi));
lt.start();
rt.start();
尝试{lt.join();}catch(InterruptedException ex){}
尝试{rt.join();}catch(I
package x;
import java.util.Random;
public class x {
class qsort0 implements Runnable
{
int[] a;
int lo;
int hi;
private qsort0(int[] a, int lo, int hi)
{
this.a = a;
this.lo = lo;
this.hi = hi;
}
@Override
public void run()
{
if(this.lo >= this.hi)
return;
int pi = partition(this.a, this.lo, this.hi);
Thread lt = new Thread(new qsort1(a, this.lo, pi));
Thread rt = new Thread(new qsort1(a, pi+1, this.hi));
lt.start();
rt.start();
try {lt.join();} catch (InterruptedException ex){}
try {rt.join();} catch (InterruptedException ex){}
}
}
class qsort1 implements Runnable
{
int[] a;
int lo;
int hi;
private qsort1(int[] a, int lo, int hi)
{
this.a = a;
this.lo = lo;
this.hi = hi;
}
@Override
public void run()
{
if(this.lo >= this.hi)
return;
int pi = partition(this.a, this.lo, this.hi);
Thread lt = new Thread(new qsort2(a, this.lo, pi));
Thread rt = new Thread(new qsort2(a, pi+1, this.hi));
lt.start();
rt.start();
try {lt.join();} catch (InterruptedException ex){}
try {rt.join();} catch (InterruptedException ex){}
}
}
class qsort2 implements Runnable
{
int[] a;
int lo;
int hi;
private qsort2(int[] a, int lo, int hi)
{
this.a = a;
this.lo = lo;
this.hi = hi;
}
@Override
public void run() {
if(this.lo >= this.hi)
return;
qsort(this.a, this.lo, this.hi);
}
}
// quicksort multi-threaded
@SuppressWarnings("empty-statement")
public static void qsortmt(int[] a, int lo, int hi)
{
if(lo >= hi)
return;
int pi = partition(a, lo, hi);
Thread lt = new Thread(new x().new qsort0(a, lo, pi));
Thread rt = new Thread(new x().new qsort0(a, pi+1, hi));
lt.start();
rt.start();
try {lt.join();} catch (InterruptedException ex){}
try {rt.join();} catch (InterruptedException ex){}
}
@SuppressWarnings("empty-statement")
public static int partition(int []a, int lo, int hi)
{
int md = lo+(hi-lo)/2;
int ll = lo-1;
int hh = hi+1;
int t;
int p = a[md];
while(true){
while(a[++ll] < p);
while(a[--hh] > p);
if(ll >= hh)
break;
t = a[ll];
a[ll] = a[hh];
a[hh] = t;
}
return hh;
}
@SuppressWarnings("empty-statement")
public static void qsort(int[] a, int lo, int hi)
{
while(lo < hi){
int ll = partition(a, lo, hi);
int hh = ll+1;
// recurse on smaller part, loop on larger part
if((ll - lo) <= (hi - hh)){
qsort(a, lo, ll);
lo = hh;
} else {
qsort(a, hh, hi);
hi = ll;
}
}
}
public static void main(String[] args)
{
int[] a = new int[16*1024*1024];
Random r = new Random(0);
for(int i = 0; i < a.length; i++)
a[i] = r.nextInt();
long bgn, end;
bgn = System.currentTimeMillis();
qsortmt(a, 0, a.length-1);
end = System.currentTimeMillis();
for(int i = 1; i < a.length; i++){
if(a[i-1] > a[i]){
System.out.println("failed");
break;
}
}
System.out.println("milliseconds " + (end-bgn));
}
}