File Occupancy in Buffer Cache

mincore.c

#include <assert.h>
#include <errno.h>
#include <fcntl.h>
#include <grp.h>
#include <pwd.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/resource.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

#define VEC_SIZE 1024
#define PG_SIZE 4096
#define RANGE_PER_MAP VEC_SIZE*PG_SIZE
#define MIN(x,y) (((x)<(y))?(x):(y))

int main(int argc, char const *argv[]) { 
	struct stat s;
	off_t my_offset;
	int size, i, nbits, j;
	long offset = 0, count = 0;
	unsigned char* addr;
	unsigned char vec[VEC_SIZE];
	int fd = open(argv[1], O_RDONLY);
	
	int status = fstat(fd, &s);
	size = s.st_size;
	off_t sz = (off_t) size;

	int npages = (sz + PG_SIZE - 1) / PG_SIZE;

	int nloops = (npages + VEC_SIZE - 1) / VEC_SIZE;
	
	my_offset = (off_t) (offset / RANGE_PER_MAP) * RANGE_PER_MAP;
	
	for(i = 0; i < nloops; ++i) {
		addr = (char*) mmap(0, RANGE_PER_MAP, PROT_READ, MAP_PRIVATE, fd, my_offset);
		memset(vec, 0, sizeof(unsigned char) * VEC_SIZE);
		nbits = MIN(sz - my_offset, RANGE_PER_MAP);
		mincore(addr, nbits, vec);
		for (j = offset > my_offset ? (offset/PG_SIZE) % VEC_SIZE : 0; j < (nbits + PG_SIZE - 1) / PG_SIZE; j++) {
			if (vec[j] & 0x1) {
				count++;
			}
		}
		my_offset += RANGE_PER_MAP;
		munmap(addr, RANGE_PER_MAP);
	}
	long sz_pages = (sz + (unsigned long long) PG_SIZE - 1) / (unsigned long long) PG_SIZE;
	printf("%s - %f\n", argv[1], (count * 100.0) / sz_pages);
	return 0;
}