Some calls to “calloc” are suspiciously fast [dupl

Question

This question already has an answer here:

• Why malloc+memset is slower than calloc? 3 answers

I'm benchmarking with “Perf” (Linux, gcc).

When allocating memory:

point_1 = calloc (100000000, 16);  //this takes nearly 1 second and perf find 27M transfers from RAM->CACHE and 1M from CACHE->RAM 

This is OK.

But when trying to allocate two arrays:

point_1 = calloc (100000000, 16); 
point_2 = calloc (100000000, 16);
  //again, program takes nearly 1 second, 27M transfers RAM-CACAHE, 1M CACHE->RAM

It looks like, that second “calloc” (and all following) are behave like “malloc”. I'm using “gcc version 4.9.2 (Ubuntu 4.9.2-0ubuntu1~12.04) ”. Otherwise program works fine.

Is that behaving OK?

Here are some more tests and results:

Time for allocating of data structure 1: 0.976468 
  Perf: R:27M, W:1M

Time for allocating of data structure 1: 0.975402 
Time for initialization of data structure 1 to value of 7: 0.296787 
  Perf: R: 52M, W: 26M

Time for allocating of data structure 1: 0.976034 
Time for initialization of data structure 1 to value of 7: 0.313554 
Time for allocating of data structure 2: 0.000031   <-- misbehaving
  Perf: R: 52M, W:26M

Time for allocating of data structure 1: 0.975403 
Time for initialization of data structure 1 to value of 7: 0.313710 
Time for allocating of data structure 2: 0.000031   <-- misbehaving
Time for initialization of data structure 2 to value of 7: 0.809855 
  Perf: R:79M, W: 53M

Answer 1

Each of the calls tries to allocate 1.6 GB of memory. I suspect the second call is failing, which would explain the symptoms. Check the return value from calloc().

Answer 2

It can confirm that the second calloc takes much shorter time. It seems that Linux decides to postpone some of the actual work.

On my system, the first calloc takes around 0.7 seconds. If I then iterate over the allocated memory area, setting it to something other than zero, this takes 0.2 seconds. In total, 0.9 seconds.

The second calloc then takes 0.0 seconds, but setting the second area takes 0.9 seconds. Same total time, but it seems that the second calloc, as Karoly Horvath wrote in a comment, doesn't actually create the memory pages, but leaves that to page faults when accessing the memory.

Another great comment by Karoly Horvath linked to this related question: Why malloc+memset is slower than calloc?

Tested on Ubuntu 14.04.1 LTS running on an Intel Core i7-4790K, with -O2 and a GCC that calls itself "gcc (Ubuntu 4.8.2-19ubuntu1) 4.8.2". Glibc version is Ubuntu EGLIBC 2.19-0ubuntu6.4.