madv.cpp

#include <unistd.h>
#include <sys/syscall.h>
#include <sys/mman.h>
#include <cstring>
#include <cstdio>

int main(int argc, char** argv) {
 constexpr size_t SIZE = 512 << 20;
 unsigned i = 10;
 void *p = mmap(NULL, SIZE, PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE, -1, 0);
 while(i--) {
   memset(p, 1, SIZE);
   // 4 = MADV_DONTNEED, 8 = MADV_FREE
   long err = syscall(SYS_madvise, p, SIZE, 4);
   if (err)
     printf("err: %ld\n", err);
 }
 return 0;
}

buchgra@linux:~$ time ./madv-dontneed 
real    0m2.599s
user    0m0.244s
sys 0m2.352s
buchgra@linux:~$ time ./madv-free 
real    0m1.384s
user    0m1.072s
sys 0m0.308s