为什么Java比C ++更快地读取大文件？ [英] Why does Java read a big file faster than C++?

问题描述

我有一个2 GB的档案（ iputfile.txt ），其中档案中的每一行都是一个字，如下：

  apple 
 red 
美丽
气味
 spark 
输入
  

我需要编写一个程序来读取文件中的每一个字并打印字数。我使用Java和C ++编写它，但结果令人惊讶：Java运行速度比C ++快2.3倍。我的代码如下：

C ++：

  ）{
 struct timespec ts，te; 
 double cost; 
 clock_gettime（CLOCK_REALTIME，& ts）; 
 
 ifstream fin（inputfile.txt）; 
 string word; 
 int count = 0; 
 while（fin>> word）{
 count ++; 
} 
 cout<< count<< endl; 
 
 clock_gettime（CLOCK_REALTIME，& te）; 
 cost = te.tv_sec  -  ts.tv_sec +（double）（te.tv_nsec-ts.tv_nsec）/ NANO; 
 printf（运行时间：％-15.10f s\\\
，cost）; 
 
 return 0; 
} 
  

输出：

  5e + 08 
运行时间：69.311 s 
  

Java：

  public static void main（String [] args）throws Exception {
 
 long startTime = System.currentTimeMillis（）; 
 
 FileReader reader = new FileReader（inputfile.txt）; 
 BufferedReader br = new BufferedReader（reader）; 
 String str = null; 
 int count = 0; 
 while（（str = br.readLine（））！= null）{
 count ++; 
} 
 System.out.println（count）; 
 
 long endTime = System.currentTimeMillis（）; 
 System.out.println（运行时间：+（endTime  -  startTime）/ 1000 +s）; 
} 
  

输出：

  5.0E8 
运行时间：29 s 
  

为什么Java在这种情况下比C ++快，而且如何提高C ++的性能？

解决方案

一样的东西。 Java程序读取行，依赖换行符，而C ++程序读取分隔单词的空格，这是一些额外的工作。

尝试<$ c $

$ b

p>您还可以尝试并执行基本读取操作以读取字节数组，并扫描此换行符。

$ b

即使以后

在我的旧Linux笔记本上，jdk1.7.0_21和don't-tell-me-it's-old 4.3.3花了大约相同的时间，与C ++ getline相比。 （我们已经确定阅读的话是更慢的。）-O0和-O2之间没有太大的区别，这并不让我惊讶，考虑到循环中的代码的简单。

最后一个注意
正如我所建议的，使用LEN = 1MB的fin.read（buffer，LEN），并使用memchr扫描'\\\
'改进约20％，这使得C（目前没有任何C ++离开）比Java快。

I have a 2 GB file (iputfile.txt) in which every line in the file is a word, just like:

apple
red
beautiful
smell
spark
input

I need to write a program to read every word in the file and print the word count. I wrote it using Java and C++, but the result is surprising: Java runs 2.3 times faster than C++. My code are as follows:

C++:

int main() {
    struct timespec ts, te;
    double cost;
    clock_gettime(CLOCK_REALTIME, &ts);

    ifstream fin("inputfile.txt");
    string word;
    int count = 0;
    while(fin >> word) {
        count++;
    }
    cout << count << endl;

    clock_gettime(CLOCK_REALTIME, &te);
    cost = te.tv_sec - ts.tv_sec + (double)(te.tv_nsec-ts.tv_nsec)/NANO;
    printf("Run time: %-15.10f s\n", cost);

    return 0;
}

Output:

5e+08
Run time: 69.311 s

Java:

 public static void main(String[] args) throws Exception {

    long startTime = System.currentTimeMillis();

    FileReader reader = new FileReader("inputfile.txt");
    BufferedReader br = new BufferedReader(reader);
    String str = null;
    int count = 0;
    while((str = br.readLine()) != null) {
        count++;
    }
    System.out.println(count);

    long endTime = System.currentTimeMillis();
    System.out.println("Run time : " + (endTime - startTime)/1000 + "s");
}

Output:

5.0E8
Run time: 29 s

Why is Java faster than C++ in this situation, and how do I improve the performance of C++?

解决方案

You aren't comparing the same thing. The Java program reads lines, depening on the newline, while the C++ program reads white space delimited "words", which is a little extra work.

Try istream::getline.

Later

You might also try and do an elementary read operation to read a byte array and scan this for newlines.

Even later

On my old Linux notebook, jdk1.7.0_21 and don't-tell-me-it's-old 4.3.3 take about the same time, comparing with C++ getline. (We have established that reading words is slower.) There isn't much difference between -O0 and -O2, which doesn't surprise me, given the simplicity of the code in the loop.

Last note As I suggested, fin.read(buffer,LEN) with LEN = 1MB and using memchr to scan for '\n' results in another speed improvement of about 20%, which makes C (there isn't any C++ left by now) faster than Java.

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