os.File.Readdirstracelstat()/usr/bin

Go code:

package main
import (
        "fmt"
    "os"
)
func main() {
    x, err := os.Open("/usr/bin")
    if err != nil {
        panic(err)
    }
    y, err := x.Readdir(0)
    if err != nil {
        panic(err)
    }
    for _, i := range y {
    fmt.Println(i)
    }

}

Strace on the program (without following threads):

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 93.62    0.004110           2      2466           write
  3.46    0.000152           7        22           getdents64
  2.92    0.000128           0      2466           lstat // this increases with increase in no. of files.
  0.00    0.000000           0        11           mmap
  0.00    0.000000           0         1           munmap
  0.00    0.000000           0       114           rt_sigaction
  0.00    0.000000           0         8           rt_sigprocmask
  0.00    0.000000           0         1           sched_yield
  0.00    0.000000           0         3           clone
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         2           sigaltstack
  0.00    0.000000           0         1           arch_prctl
  0.00    0.000000           0         1           gettid
  0.00    0.000000           0        57           futex
  0.00    0.000000           0         1           sched_getaffinity
  0.00    0.000000           0         1           openat
------ ----------- ----------- --------- --------- ----------------
100.00    0.004390                  5156           total

readdir()

C code:

#include <stdio.h>
#include <dirent.h>

int main (void) {
    DIR* dir_p;
    struct dirent* dir_ent;

    dir_p = opendir ("/usr/bin");

    if (dir_p != NULL) {
        // The readdir() function returns a pointer to a dirent structure representing the next
        // directory entry in the directory stream pointed to by dirp.
        // It returns NULL on reaching the end of the directory stream or if an error occurred.
        while ((dir_ent = readdir (dir_p)) != NULL) {
            // printf("%s", dir_ent->d_name);
            // printf("%d", dir_ent->d_type);
            if (dir_ent->d_type == DT_DIR) {
                printf("%s is a directory", dir_ent->d_name);
            } else {
                printf("%s is not a directory", dir_ent->d_name);
            }

            printf("
");
        }
            (void) closedir(dir_p);

    }
    else
        perror ("Couldn't open the directory");

    return 0;
}

Strace on the program:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
100.00    0.000128           0      2468           write
  0.00    0.000000           0         1           read
  0.00    0.000000           0         3           open
  0.00    0.000000           0         3           close
  0.00    0.000000           0         4           fstat
  0.00    0.000000           0         8           mmap
  0.00    0.000000           0         3           mprotect
  0.00    0.000000           0         1           munmap
  0.00    0.000000           0         3           brk
  0.00    0.000000           0         3         3 access
  0.00    0.000000           0         1           execve
  0.00    0.000000           0         4           getdents
  0.00    0.000000           0         1           arch_prctl
------ ----------- ----------- --------- --------- ----------------
100.00    0.000128                  2503         3 total

I am aware that the only fields in the dirent structure that are mandated by POSIX.1 are d_name and d_ino, but I am writing this for a specific filesystem. 

*File.Readdirnames()lstatlstat
lstat()open("/usr/bin", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFDIR|0755, st_size=69632, ...}) = 0
brk(NULL)                               = 0x1098000
brk(0x10c1000)                          = 0x10c1000
getdents(3, /* 986 entries */, 32768)   = 32752CGO