Transparent Huge Page (THP) is a technique to take advantage of large-page (>4K) without (or with little) app modification. Here is a simple C program to illustrate how THP can be used and the effect of different huge-page allocation policy.

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#include <assert.h>
#include <sys/mman.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <stdio.h>

int main(void) {
  unsigned char vec[20];
  int res;
  size_t PS = (size_t)sysconf(_SC_PAGESIZE);
  size_t ps_2m = (1 << 21);

  void* addr;
  // make sure it's 2M-aligned
  posix_memalign(&addr, ps_2m, ps_2m);
  printf("addr: %p\n", addr);
  puts("");

  // depending on the huge-page alloc policy:
  //   /sys/kernel/mm/transparent_hugepage/enabled
  //   always:  no need for madvise
  //   madvise: needed
  //   never:   no effect either way
  //
  // res = madvise(addr, ps_2m, MADV_HUGEPAGE);
  // assert(res == 0);

  res = mincore(addr, 10 * PS, vec);
  assert(res == 0);

  puts("not committed");
  for (int i = 0; i < 10; ++i) {
    printf("%d", (vec[i] & 1));
  }

  puts("");

  // write to the first byte
  ((char *)addr)[0] = 1;

  res = mincore(addr, 10 * PS, vec);
  assert(res == 0);

  puts("");
  puts("after writing the first byte");
  for (int i = 0; i < 10; ++i) {
    printf("%d", (vec[i] & 1));
  }

  puts("");

  return 0;
}

Using the always policy, echo always > /sys/kernel/mm/transparent_hugepage/enabled,:

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addr: 0x7fcf0ae00000

not committed
0000000000

after writing the first byte
1111111111