Question

我想写一下Eratosthenes的筛子，它将使用特定数量的线程。我发现，它将以下列方式工作：适用于最多17个的2个线程。 Thread-1取2，并开始从List中删除2的多个。 Parallel Thread-2取3并且做同样的事情。之后，Thread-1需要5（因为List中没有4）而Thread-2需要7，依此类推，直到它们到达结束。我写了以下代码：

private List<Integer> array = new ArrayList<Integer>();
private List<Integer> results = new ArrayList<Integer>();
public synchronized void run(){
    while(array.size() > 0){
        Integer tmp = array.get(0);
        for(int i = 1; i < array.size(); i++){
            if( (array.get(i).intValue() % tmp.intValue()) == 0)
                array.remove(i);
        }
        results.add(array.get(0));
        array.remove(0);
    }
}

public void setArray(int x){
    for(int i = 2; i < x; i++)
        array.add(Integer.valueOf(i));
}
public void printArray(){
    for(Integer i: results){
        System.out.println(i);
    }
}

此代码有效，但我将时间测量“工具”添加到我的主要课程中：

ThreadTask task = new ThreadTask();
task.setArray(5000);
Long beg = new Date().getTime();
for(int i = 0; i < 3;i++){
    new Thread(task).start();
}
Long sleep = 1000L;
Thread.sleep(sleep);// I am sleeping main thread to wait until other Threads are done
task.printArray();
System.out.println("Time is "+(new Date().getTime()-beg-sleep));

问题是用2个线程运行它比用1个线程运行慢，3个线程慢于2个线程。谁能解释我，为什么？

修改

有一件重要的事情。我不需要尽可能快地完成它。我需要它在Threads上工作有一个原因。我的老师想比较运行相同程序的运行时与1,2 ... n个线程。结果应与this图表类似。

EDIT2：

我已将代码重写为以下

private HashMap<Integer,Boolean> array = new HashMap<Integer,Boolean>();
private int counter = 1;
private int x;
public void run(){
    while(counter < x-1){
        do{
            counter++;
        }
        while( array.get(counter));
        int tmp = counter;
        for(int i = tmp; i < array.size(); i+=tmp){
            if( i!= tmp)
                array.put(i,true);
        }
        try{
        Thread.sleep(0L, 1);
        }
        catch (Exception e){}
    }
}

public void setArray(int x){
    this.x = x;
    for(int i = 2; i < x; i++)
        array.put(i, false);
}
public void printArray(){

    for(int i = 2; i < array.size();i++){
        if( !array.get(i))
        System.out.println(i);

    }
}

现在它使用HashMap，这就是它的工作原理：

使用从2到n的键和错误值填充HashMap。
新线程进入基于counter变量的while循环。 Counter代表当前密钥。
在乞讨时增加计数器，因此新线程不会在先前启动的线程的counter上运行。
将counter值放入临时变量tmp中，这样即使其他线程增加counter
通过使用i递增tmp来迭代HashMap（它实际上跳过i的乘法）并将其值设置为true。
打印方法中忽略具有true值的所有键。同时counter会在递增时跳过它们。

问题是它仍然可以使用更多线程更慢。现在怎么了？

Answer 1

这个错误比我原先想象的要简单。你所有的线程都在做同样的事情，所以每个线程都做得更多。为了使多线程程序更快地工作，你必须将工作分开，这必须同时执行。

当你有一个线程访问数据结构时，它可以在一个核心的最快缓存中，使用多个线程，他们需要协调他们的操作，因为大多数工作是更新数据结构，很多时间浪费作为开销。即使您的数据结构不是线程安全的，并且结果可能已损坏，也是如此。

BTW更新ArrayList非常昂贵，使用集合对象也是一种开销。

使用BitSet和一个线程，您将获得更快的结果。

public class BitSetSieveMain {
    private final BitSet set;
    private final int size;

    public BitSetSieveMain(int x) {
        size = x + 1;
        set = new BitSet(size);
        set.flip(2, size);
    }

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            long start = System.nanoTime();
            BitSetSieveMain bitSetSieveMain = new BitSetSieveMain(5000);
            bitSetSieveMain.sieve();
            long time = System.nanoTime() - start;
            System.out.println(time / 1000 + " micro-seconds to perform " + bitSetSieveMain);
        }
    }

    public void sieve() {
        int i = 2;
        do {
            for (int j = i*2; j < size; j += i)
                set.clear(j);
            i = set.nextSetBit(i+1);
        } while (i > 0);
    }

    public String toString() {
        return set.toString();
    }
}

终于打印

87微秒执行{2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71， 73,79,83,89,97,101,103,107,109,113,127,131,137,139,149,151,157,163,167,173,179,181,191,193,197， 199,211,223,227,229,233,239,241,251,257,263,269,271,277,281,283,293,307,311,313,317,331,337,347,349， 353,359,367,373,379,383,389,397,401,409,419,421,431,433,439,443,449,457,461,463,467,479,487,491,499， 503,509,521,523,541,547,557,563,569,571,577,587,593,599,601,607,613,617,619,631,641,643,647,653,659， 661,673,677,683,691,701,709,719,727,733,739,743,751,757,761,769,773,787,797,809,811,821,823,827,829， 839,853,857,859,863,877,881,883,887,907,911,919,929,937,941,947,953,967,971,977,983,991,997,1009,1013， 1019,1021,1031,1033,1039,1049,1051,1061,1063,1069,1087,1091,1093,1097,1103,1109,1117,1123,1129,1151,1153,1163,1171,1181,1 187,1193,1201,1213,1217,1223,1229,1231,1237,1249,1259,1277,1279,1283,1289,1291,1297,1301,1303,1307,1319,1321,1327,1361,1367， 1373,1381,1399,1409,1423,1427,1429,1433,1439,1447,1451,1453,1459,1471,1481,1483,1487,1489,1493,1499,1511,1523,1531,1543,1549， 1553,1559,1567,1571,1579,1583,1597,1601,1607,1609,1613,1619,1621,1627,1637,1657,1663,1667,1669,1693,1697,1699,1709,1721,1723， 1733,1741,1747,1753,1759,1777,1783,1787,1789,1801,1811,1823,1831,1847,1861,1867,1871,1873,1877,1879,1889,1901,1907,1913,1931， 1933,1949,1951,1973,1979,1987,1993,1997,1999,2003,2011,2017,2027,2029,2039,2053,2063,2069,2081,2083,2087,2089,2099,2111,2113， 2129,2131,2137,2141,2143,2153,2161,2179,2203,2207,2213,2221,2237,2239,2243,2251,2267,2269,2273,2281,2287,2293,2297,2309,2311， 2333,2339,2341,2347,2351,2357,2371,2377,2381,2383,2389,2393,2399,2411,2417,2423,2437， 2441,2447,2459,2467,2473,2477,2503,2521,2531,2539,2543,2549,2551,2557,2579,2591,2593,2609,2617,2621,2633,2647,2657,2659,2663， 2671,2677,2683,2687,2689,2693,2699,2707,2711,2713,2719,2729,2731,2741,2749,2753,2767,2777,2789,2791,2797,2801,2803,2819,2833， 2837,2843,2851,2857,2861,2879,2887,2897,2903,2909,2917,2927,2939,2953,2957,2963,2969,2971,29999,3001,3011,3019,3023,3037,3041， 3049,3061,3067,3079,3083,3089,3109,3119,3121,3177,3163,3167,3169,3181,3177,3191,3203,3209,3217,3221,3229,3251,3253,3257,3259， 3271,3299,3301,3307,3313,3319,3323,3329,3331,3333,3347,3359,3361,3371,3333,3389,3391,3407,3413,3433,3449,3457,3461,3463,3467， 3469,3491,3499,3511,3517,3527,3529,3533,3539,3541,3547,3557,3559,3571,3581,3583,3593,3607,3613,3617,3623,3631,3637,3643,3659， 3671,3673,3677,3691,3677,3701,3799,3719,3727,3733,3739,3761,3767,3769,3779,3793,379 7,3803,3821,3823,3833,3847,3851,3853,3863,3877,3881,3889,3907,3911,3917,3919,3923,3929,3931,3943,3947,3967,3989,4001,4003， 4007,4013,4019,4027,4027,4049,4051,4057,4073,4079,4091,4093,4099,4111,4127,4129,4133,4139,4153,4157,4159,4177,4201,4211,4217， 4219,4229,4231,4241,4243,4253,4259,4261,4271,4273,4283,4289,4297,4327,4337,4399,4359,4357,4363,4373,4391,4397,4409,4421,4423， 4441,4447,4451,4457,4463,4481,4483,4493,4507,4513,4517,4519,4523,4547,4549,4561,4567,4583,4591,4597,4603,4621,4637,4639,4643， 4649,4651,4657,4663,4673,4679,4691,4703,4721,4723,4724,4733,4751,4759,4783,4787,4789,4793,4799,4801,4813,4817,4831,4861,4871， 4877,4889,4903,4909,4919,4931,4933,4937,4943,4951,4957,4967,4969,4973,4987,4993,4999}

Eratosthenes平行筛 - Java多线程

1 个答案: