I am running a test to check disk read statistics. Here is the code for the same:
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
int main(int argc, char* argv)
{
int count=1000, size;
char block[4096]="0";
int fd = open("file1.txt",O_RDONLY | O_SYNC);
//int pid = getpid();
system("pid=$(ps -a | grep 'a.out' | awk '{print $1}'); iotop -bokp $pid > test1c.out &");
system("echo 'Starts reading in 10'");
srand(time(NULL));
system("sleep 1");
while(count--){
int random = (rand()%16)*666;
printf("%d;",random);
lseek(fd, random, SEEK_SET);
size = read(fd,block,4096);
printf("Number of bytes read: %d\n", size);
fsync(fd);
//printf("Read 4kb from the file.\n");
}
system("sleep 1");
system("killall iotop");
}
As you can see, I am opening a large file, getting the PID of my a.out file, and passing it to iotop. After that I am randomly seeking to a 4kb block in the file and reading data.
If you run this code on your system, you'll realize that iotop output shows 0kb reads throughout, which makes no sense. Am I doing something wrong?
Clearing the caches solved the issue. I found the script for clearing caches on this page:
https://www.tecmint.com/clear-ram-memory-cache-buffer-and-swap-space-on-linux/
sync; echo 1 > /proc/sys/vm/drop_caches
sync; echo 2 > /proc/sys/vm/drop_caches
sync; echo 3 > /proc/sys/vm/drop_caches
Does the trick!