我的第一个循环读取一些大文本文件并计算行/行.
在malloc之后,第二个循环填充分配的矩阵.

如果第二个循环被注释掉,则第一个循环需要1.5秒.但是,使用第二个循环进行编译会减慢第一个循环,现在需要30-40秒！

换句话说：第二个循环以某种方式减慢了第一个循环.我试图改变范围,更改编译器,更改编译器标志,更改循环本身,将所有内容放入main(),使用boost :: iostream甚至在共享库中放置一个循环,但在每次尝试中都会出现同样的问题！

第一个循环很快,直到用第二个循环编译程序.

编辑：这是我的问题的完整示例————>

#include <iostream>
#include <vector>
#include "string.h"
#include "boost/chrono.hpp"
#include "sys/mman.h"
#include "sys/stat.h"
#include "fcntl.h"
#include <algorithm>

unsigned long int countLines(char const *fname) {
    static const auto BUFFER_SIZE = 16*1024;
    int fd = open(fname,O_RDONLY);
    if(fd == -1) {
        std::cout << "Open Error" << std::endl;
        std::exit(EXIT_FAILURE);
    }

    posix_fadvise(fd,1); 
    char buf[BUFFER_SIZE + 1];
    unsigned long int lines = 0;

    while(size_t bytes_read = read(fd,buf,BUFFER_SIZE)) {
        if(bytes_read == (size_t)-1) {
            std::cout << "Read Failed" << std::endl;
            std::exit(EXIT_FAILURE);
        }
        if (!bytes_read)
            break;

        int n;
        char *p;
        for(p = buf,n=bytes_read ; n > 0 && (p = (char*) memchr(p,'\n',n)) ; n = (buf+bytes_read) - ++p)
            ++lines;
    }
    close(fd);
    return lines;
}

int main(int argc,char *argv[])
{
    // initial variables
    int offset = 55;  
    unsigned long int rows = 0;
    unsigned long int cols = 0;
    std::vector<unsigned long int> dbRows = {0,0};
    std::vector<std::string> files = {"DATA/test/file1.csv",// large files: 3Gb 
                                      "DATA/test/file2.csv",// each line is 55 chars long 
                                      "DATA/test/file3.csv"};

    // find each file's number of rows 
    for (int x = 0; x < files.size(); x++) {   // <--- FIRST LOOP **
        dbRows[x] = countLines(files[x].c_str());
    }

    // define matrix row as being the largest row found 
    // define matrix col as being 55 chars long for each csv file
    std::vector<unsigned long int>::iterator maxCount;
    maxCount = std::max_element(dbRows.begin(),dbRows.end());
    rows = dbRows[std::distance(dbRows.begin(),maxCount)];   // typically rows = 72716067
    cols = dbRows.size() * offset;                            //           cols = 165 

    // malloc required space (11998151055)
    char *syncData = (char *)malloc(rows*cols*sizeof(char));

    // fill up allocated memory with a test letter
    char t[]= "x";
    for (unsigned long int x = 0; x < (rows*cols); x++) {   // <--- SECOND LOOP **
        syncData[x] = t[0];
    } 

    free(syncData);
    return 0; 
}

我还注意到,降低列数会加快第一个循环的速度.

探查器将手指指向此行：

while(size_t bytes_read = read(fd,BUFFER_SIZE))

该程序在此行上空闲30秒或等待计数为230,000.
在汇编中,等待计数发生在：

Block 5:
lea 0x8(%rsp),%rsi
mov %r12d,%edi
mov $0x4000,%edx
callq  0x402fc0     <------ stalls on callq
Block 6:
mov %rax,%rbx
test %rbx,%rbx
jz 0x404480 <Block 18>

我的猜测是从流中读取时出现了一个API块,但我不知道为什么？