开学了，出个题消遣下_梁斌

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开学了，出个题消遣下

(2012-02-20 10:56:35)

标签：

杂谈

linux提供的读文件函数，如果读一个较大的文件，总会间隔的出现行读变慢的情况。这是什么原因呢？怎么避免呢？呵呵。第一个完整答对者，我在微博上加他一声大哥吧（我可出不起iphone哦)。奖励只限在校学生哦。

例如：注意第1460行和1487行的耗时

1458th the time cost for read a line 164

1459th the time cost for read a line 844

1460th the time cost for read a line 6016

1461th the time cost for read a line 232

1462th the time cost for read a line 232

1463th the time cost for read a line 240

1464th the time cost for read a line 192

1465th the time cost for read a line 788

1466th the time cost for read a line 264

1467th the time cost for read a line 392

1468th the time cost for read a line 176

1469th the time cost for read a line 276

1470th the time cost for read a line 324

1471th the time cost for read a line 312

1472th the time cost for read a line 252

1473th the time cost for read a line 260

1474th the time cost for read a line 196

1475th the time cost for read a line 292

1476th the time cost for read a line 356

1477th the time cost for read a line 260

1478th the time cost for read a line 284

1479th the time cost for read a line 208

1480th the time cost for read a line 344

1481th the time cost for read a line 316

1482th the time cost for read a line 304

1483th the time cost for read a line 356

1484th the time cost for read a line 264

1485th the time cost for read a line 268

1486th the time cost for read a line 224

1487th the time cost for read a line 5276

1488th the time cost for read a line 380

1489th the time cost for read a line 332

1490th the time cost for read a line 328

以下为实验代码

#include <stdio.h>

#include <stdlib.h>

#if defined(__i386__)

static __inline__ unsigned long long rdtsc(void)

{

unsigned long long int x;

__asm__ volatile ("rdtsc" : "=A" (x));

return x;

}

#elif defined(__x86_64__)

static __inline__ unsigned long long rdtsc(void)

{

unsigned hi, lo;

__asm__ __volatile__ ("rdtsc" : "=a"(lo), "=d"(hi));

return ( (unsigned long long)lo)|( ((unsigned long long)hi)<<32 );

}

#endif

int main(void)

{

FILE * fp;

char * line = NULL;

size_t len = 0;

ssize_t read;

fp = fopen("./some_file_with_many_line", "r");

if (fp == NULL)

exit(EXIT_FAILURE);

int start;

int end;

int i=0;

start = rdtsc();

while ((read = getline(&line, &len, fp)) != -1) {

end = rdtsc();

printf("%dth\t the time cost for read a line %d\n",i++,end - start);

start = rdtsc();

}

if (line)

free(line);

exit(EXIT_SUCCESS);

}

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