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POCan some explain the performance behavior of the following memory allocating C program?
 

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  1. POCan some explain the performance behavior of the following memory allocating C program?
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    <p>On my machine Time A and Time B swap depending on whether <code>A</code> is defined or not (which changes the order in which the two <code>calloc</code>s are called).</p> <p>I initially attributed this to the paging system. Weirdly, when <code>mmap</code> is used instead of <code>calloc</code>, the situation is even more bizzare -- both the loops take the same amount of time, as expected. As can be seen with <code>strace</code>, the <code>calloc</code>s ultimately result in two <code>mmap</code>s, so there is no return-already-allocated-memory magic going on.</p> <p>I'm running Debian testing on an Intel i7.</p> <pre><code>#include &lt;stdlib.h&gt; #include &lt;stdio.h&gt; #include &lt;sys/mman.h&gt; #include &lt;time.h&gt; #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 &lt; 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 &lt; SIZE; i++) arr2[i] = (i + 13) * 5; } finish = clock(); printf("Time B: %.2f\n", ((double)(finish - start))/CLOCKS_PER_SEC); return 0; } </code></pre> <p>The output I get:</p> <pre><code> ~/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 </code></pre>
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      1. POCan some explain the performance behavior of the following memory allocating C program?
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    1. COYou should pose your question (clearly) outside the code block. Hiding it in code comments is not helpful.
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      1. POCan some explain the performance behavior of the following memory allocating C program?
      1. USDaniel Fischer
    2. COMight want to drop the C++ tag in favor of *nix. Can you be a bit more specific in what you are looking for? Basically it either uses a memory mapped file or regular allocation ...
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      1. POCan some explain the performance behavior of the following memory allocating C program?
      1. USAJG85
    3. CO+1: I have encountered the exact same phenomenon when doing performance measurements before. I never managed to figure out what was going on, so I really hope you get a good answer.
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