Is Linux perf imprecise for page fault and tlb miss?

Question

I wrote a simple program to test page faults and tlb miss with perf. The code is as follow. It writes 1 GB data sequentially and is expected to trigger 1GB/4KB=256K tlb misses and page faults.

#include<stdio.h>
#include <stdlib.h>

#define STEP 64
#define LENGTH (1024*1024*1024)
int main(){
    char* a = malloc(LENGTH);
    int i;
    for(i=0; i<LENGTH; i+=STEP){
            a[i] = 'a';
    }

    return 0;
}

However, the result is as follow and far smaller than expected. Is perf so imprecise? I would be very appreciated if anyone can run the code on his machine.

 $ perf stat -e dTLB-load-misses,page-faults ./a.out

   Performance counter stats for './a.out':

         12299      dTLB-load-misses
          1070      page-faults

   0.427970453 seconds time elapsed

Environment: Ubuntu 14.04.5 LTS , kernel 4.4.0; gcc 4.8.4 glibc 2.19. No compile flags.

The CPU is Intel(R) Xeon(R) CPU E5-2640 v2 @ 2.00GHz.

Do you have transparent huge pages enabled? This might be a reason for lower TLB-load-misses. — andrjas, Nov 29 '17 at 08:04
No. Actually, I wanted to use huge page but fount it does not help, so I did the profile and found that 4KB page already worked well. — Waker Leo, Nov 29 '17 at 08:13
it's better to copy text output from your program and paste here instead of capturing a screenshot. And you [don't need to cast the result of malloc in C](https://stackoverflow.com/q/605845/995714) — phuclv, Nov 29 '17 at 08:28
I can't reproduce the low number of page faults. Please provide more system details (versions of distribution, kernel, glibc, compiler (flags)). — Zulan, Nov 29 '17 at 23:15

score 0 · Answer 1 · answered Nov 29 '17 at 08:57

The kernel prefetches pages on a fault, at least after it has evidence of a pattern. Can't find a definitive reference on the algorithm, but perhaps https://github.com/torvalds/linux/blob/master/mm/readahead.c is a starting point to seeing what is going on. I'd look for other performance counters that capture the behavior of this mechanism.

Is Linux perf imprecise for page fault and tlb miss?

1 Answers1