Get page cache statistics for files

pcstat.cpp

#include <cstdio>
#include <cstdint>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
    if (argc == 1) {
        fprintf(stderr, "usage: pcstat <file>");
        return 1;
    }

    printf("name:\t%s\n", argv[1]);

    struct stat fs {};

    if (stat(argv[1], &fs) != 0) {
        perror("error stat file");
        return 1;
    }

    size_t file_size = fs.st_size;
    size_t page_size = getpagesize();

    printf("size:\t%zu\n", file_size);

    size_t pages = (file_size + (page_size - 1)) / page_size;

    printf("pages:\t%zu\n", pages);

    int fd = open(argv[1], O_RDONLY);

    if (fd == -1) {
        perror("error open file");
        return 1;
    }

    void *base = mmap(nullptr, pages * page_size, PROT_READ, MAP_PRIVATE, fd, 0);

    auto *ptr = new uint8_t[pages * page_size];
    mincore(base, pages * page_size, ptr);

    size_t cached = 0;

    for (size_t i = 0; i < pages; i++) {
        if (ptr[i] & 1) {
            cached++;
        }
    }

    delete[] ptr;

    printf("cached:\t%zu (%.2f%%)\n", cached, (float) cached / (float) pages * 100);

    return 0;
}