Bandwidth readings increase while writing large file

Question

We've been measuring the bandwidth of two external HDDs in class using gettimeofday.

Surprisingly enough, and after several repeats (three measurements per execution, executed three times), we found that writing a 2500MB file was faster than writing a smaller one on both HDDs.

This is our C code. It is called from a python script to generate some charts.

//argv[1] = path, argv[2] = size in MB (2500 in this case)

#include <stdio.h>
#include <sys/time.h>
#include <time.h>
#include <unistd.h>
#include <fcntl.h>


struct timeval tv0;
struct timeval tv1;

int main(int argc, char *argv[]){
    unsigned long size=atoi(argv[2])*1000L*1000L;
    int f = open(argv[1], O_CREAT|O_WRONLY|O_TRUNC, 0777);
    char * array = malloc(size);
    gettimeofday(&tv0, 0); //START TIME
    write(f, array, size);
    fdatasync(f);
    close(f);
    gettimeofday(&tv1, 0); // END TIME 
    double seconds = (((double)tv1.tv_sec*1000000.0 + (double)tv1.tv_usec) - ((double)tv0.tv_sec*1000000.0 + (double)tv0.tv_usec))/1000000.0;
    printf("%f",seconds);
}

The teacher didn't know, so I'm asking here: is there a reason why this might happen?

Answer 1

您的基准存在严重缺陷：

• 假设没有检查所有函数调用是否成功而没有错误。
• 它假定成功时，write()将写入指定给它的完整字节数，但绝不保证这样做。

如果您的假设不满足，那么其中任何一个都很容易使您的基准测试结果无效，并且至少第二个很可能会以这种方式结束。

特别注意write()返回写入的字节数，为ssize_t。 ssize_t是有符号整数类型，其特定宽度取决于系统。如果你的大小是32位，那么write() 不能在一次调用中写入所有2500MB缓冲区，因为这比有符号的32位整数可以表示的字节多（限制为略高于2100 MB）。

此外，您的程序假定它可以成功分配非常大的内存块，这可能很容易变成不是这种情况。但是，如果这个假设失败了，你可能会因崩溃而获得奖励。