Why is std::vector Faster than std::array in Iteration?

Performance Differential between Iteration through std::array and std::vector

My previous benchmark, which sought to evaluate the iterative performance of std::array and std::vector, contained several imperfections. Upon revisiting the issue, however, I discovered that std::vector is, in fact, faster than std::array.

To alleviate any potential for optimization, I employed several measures, including:

• Utilizing the loop's result to prevent compiler optimizations
• Compiling with the -O3 flag for enhanced speed
• Measuring exclusively the targeted for loop using std::chrono

The resulting execution times for both data structures are as follows:

std::array:

$ g++ arrVsVec.cpp -O3
$ ./a.out
result: 0
time: 99.554109

std::vector:

$ g++ arrVsVec.cpp -O3
$ ./a.out
result: 0
time: 30.734491

Reason for the Discrepancy:

The performance discrepancy stems from the memory pages of the std::array not being resident in the process address space. Specifically, a global scope std::array resides in the .bss section of the executable, which has not been paged in and is initialized to zero. On the other hand, the std::vector has been allocated and initialized to zero, resulting in its memory pages already being present in the address space.

Resolving the Issue:

To eliminate this issue and demonstrate parity between the two data structures, one can add the following code as the first line of main to bring the pages in:

<code class="cpp">std::fill_n(v.data(), n, 1); // included in <algorithm></code>

Alternatively, on Linux systems, one can use mlock(v.data(), v.size() * sizeof(v[0])); to force the pages into the address space. Refer to the man mlock manual page for further details.

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