Hugetlpages Vs small pages

[Sen Horak <sen.horak@gmail.com>]

Hi,

My config: Pentium M/1400MHz/1MB L2/64 entry D-TLB
Kernel: 2.6.7-vanilla

I tried running a microbenchmark in which I "touched" every 4k in a
loop in 2 separate 4MB regions:

1)  mapped from a hugetlbfs partition (MAP_FIXED)
2)  mapped anonymously (MAP_ANONYMOUS)

The result I'm getting is that both seem to be delivering the same
performance, with an edge for MAP_ANONYMOUS.

>From hugetlbfs:   3515154447  cycles
MAP_ANONYMOUS:  3461527548 cycles
[using rdtsc]

samples  %        symbol name
630      100.000  main
samples  %        symbol name
624      100.000  main
[using oprofile]

Can somebody explain why this is happening? Unfortunately, there is no
hardware performance counter that allows one to measure D-TLB misses
on the Pentium,  so I can't measure that.

Regards,
Sen

void *init_huge_page(int size) {
    fd = open("/mnt/superpage/x", O_CREAT|O_RDWR, 0755);
    if (fd < 0) {
            perror("Open failed");
            exit(errno);
    }
    addr = (unsigned long) mmap(0x80000000, 4*1024*1024,
(PROT_READ|PROT_WRITE), MAP_SHARED|MAP_FIXED, fd, 0);
    if (addr==-1) {
            perror("mmap failed");
            exit(errno);
    }
    else
        printf("Address=%x\n",addr);
    return (void *) addr;
}

void exit_huge_page(int size) {
    if (addr!=-1) munmap(addr,size);
    close(fd);
}


int main(int argc,char *argv[])
{
  int j,i;
  unsigned long cnt1,cnt2;

  void *p=init_huge_page(BUFSIZE);

  setpriority(PRIO_PROCESS, 0, -20);
  rdtsc(cnt1);
  for(j=0;j<1000;j++) {
      for(i=0;i<(4*1024*1024);i+=1024) {
          ((char *)p)[i]='H';
      }
  }
  rdtsc(cnt2);
  printf("%0.2f\n",(cnt2-cnt1)/1000000000.0);
  exit_huge_page(BUFSIZE);