Question

On my machine Time A and Time B swap depending on whether A is
defined or not (which changes the order in which the two callocs are called).

I initially attributed this to the paging system. Weirdly, when
mmap is used instead of calloc, the situation is even more bizzare — both the loops take the same amount of time, as expected. As
can be seen with strace, the callocs ultimately result in two
mmaps, so there is no return-already-allocated-memory magic going on.

I’m running Debian testing on an Intel i7.

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

#include <time.h>

#define SIZE 500002816

#ifndef USE_MMAP
#define ALLOC calloc
#else
#define ALLOC(a, b) (mmap(NULL, a * b, PROT_READ | PROT_WRITE,  \
                          MAP_PRIVATE | MAP_ANONYMOUS, -1, 0))
#endif

int main() {
  clock_t start, finish;
#ifdef A
  int *arr1 = ALLOC(sizeof(int), SIZE);
  int *arr2 = ALLOC(sizeof(int), SIZE);
#else
  int *arr2 = ALLOC(sizeof(int), SIZE);
  int *arr1 = ALLOC(sizeof(int), SIZE);
#endif
  int i;

  start = clock();
  {
    for (i = 0; i < SIZE; i++)
      arr1[i] = (i + 13) * 5;
  }
  finish = clock();

  printf("Time A: %.2f\n", ((double)(finish - start))/CLOCKS_PER_SEC);

  start = clock();
  {
    for (i = 0; i < SIZE; i++)
      arr2[i] = (i + 13) * 5;
  }
  finish = clock();

  printf("Time B: %.2f\n", ((double)(finish - start))/CLOCKS_PER_SEC);

  return 0;
}

The output I get:

 ~/directory $ cc -Wall -O3 bench-loop.c -o bench-loop
 ~/directory $ ./bench-loop 
Time A: 0.94
Time B: 0.34
 ~/directory $ cc -DA -Wall -O3 bench-loop.c -o bench-loop
 ~/directory $ ./bench-loop                               
Time A: 0.34
Time B: 0.90
 ~/directory $ cc -DUSE_MMAP -DA -Wall -O3 bench-loop.c -o bench-loop
 ~/directory $ ./bench-loop                                          
Time A: 0.89
Time B: 0.90
 ~/directory $ cc -DUSE_MMAP -Wall -O3 bench-loop.c -o bench-loop 
 ~/directory $ ./bench-loop                                      
Time A: 0.91
Time B: 0.92

Answer 1

Short Answer

The first time that calloc is called it is explicitly zeroing out the memory. While the next time that it is called it assumed that the memory returned from mmap is already zeroed out.

Details

Here’s some of the things that I checked to come to this conclusion that you could try yourself if you wanted:

1. Insert a calloc call before your first ALLOC call. You will see that after this the Time for Time A and Time B are the same.

2. Use the clock() function to check how long each of the ALLOC calls take. In the case where they are both using calloc you will see that the first call takes much longer than the second one.

3. Use time to time the execution time of the calloc version and the USE_MMAP version. When I did this I saw that the execution time for USE_MMAP was consistently slightly less.

4. I ran with strace -tt -T which shows both the time of when the system call was made and how long it took. Here is part of the output:

Strace output:

21:29:06.127536 mmap(NULL, 2000015360, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fff806fd000 <0.000014>
21:29:07.778442 mmap(NULL, 2000015360, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fff093a0000 <0.000021>
21:29:07.778563 times({tms_utime=63, tms_stime=102, tms_cutime=0, tms_cstime=0}) = 4324241005 <0.000011>

You can see that the first mmap call took 0.000014 seconds, but that about 1.5 seconds elapsed before the next system call. Then the second mmap call took 0.000021 seconds, and was followed by the times call a few hundred microsecond later.

I also stepped through part of the application execution with gdb and saw that the first call to calloc resulted in numerous calls to memset while the second call to calloc did not make any calls to memset. You can see the source code for calloc here (look for __libc_calloc) if you are interested. As for why calloc is doing the memset on the first call but not subsequent ones I don’t know. But I feel fairly confident that this explains the behavior you have asked about.

As for why the array that was zeroed memset has improved performance my guess is that it is because of values being loaded into the TLB rather than the cache since it is a very large array. Regardless the specific reason for the performance difference that you asked about is that the two calloc calls behave differently when they are executed.