io_exerciser.c

//Invoke like `./bin <frequency in Hz (e.g. 100)> <filename to read>`

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdbool.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <unistd.h>

#define DEVICE_NAME "/sys/block/nvme0n1/stat"

unsigned long get_io_ticks() {
	char b[4096];
	char * buf = b;
	bzero(buf, 4096);
	int fd = open(DEVICE_NAME, O_RDONLY);
	read(fd, buf, 4096);
	close(fd);
	
	for(int i=0; i<9; i++) {
		buf += strspn(buf, " ");
		buf = strchr(buf, ' ')+1;
	}
	buf += strspn(buf, " ");
	char * delim = strchr(buf, ' ');
	*delim = 0;
	return atoi(buf) / 10;
}

int main(int argc, char ** argv) {
	struct timespec ts;

	int interval = 1000 * 1000 / atoi(argv[1]);
	
	time_t stamp = 0;
	int interval_stamp = 0;
	int slept = 0;
	ulong saved_ticks = 0;
	int fd = open(argv[2], O_DIRECT | O_RDONLY);
	if(fd < 0) {
		perror("open");
		exit(1);
	}
	void * buf = valloc(4096);
	while(true) {
		if(pread(fd, buf, 4096, 0) < 0) {
			perror("pread");
			exit(1);
		}
		clock_gettime(CLOCK_MONOTONIC_RAW, &ts);
		int uclock = ts.tv_nsec / 1000;
		int sleep;
		if(uclock / interval > interval_stamp) {
			sleep = 0;
		} else {
			sleep = interval - uclock % interval;
		}
		interval_stamp = (uclock / interval) + 1;

		if(stamp != ts.tv_sec) {
			unsigned long ticks = get_io_ticks();
			printf(
				"disk read time: %d%%, kernel util: %lu%%\n",
				100 - slept / 10000, ticks - saved_ticks
			);
			saved_ticks = ticks;
			stamp = ts.tv_sec;
			slept = 0;
		}
		if(sleep > 0) {
			slept += sleep;
			usleep(sleep);
		}
	}
}