


Why is Iterating through `std::vector` Faster than `std::array` in This Benchmark?
Nov 01, 2024 pm 02:50 PMThe Perplexity of Iteration Performance: std::array vs. std::vector
In a previous inquiry, it was discovered that iterating through an std::array was noticeably faster than iterating through an std::vector. To rectify these flawed observations, a new benchmark was devised, revealing a startling reversal: iterating through std::vector was now significantly faster.
To avoid potential optimizations, the result of the loop was included, and the -O3 flag was enabled. Additionally, std::chrono was used for precise timing, isolating the execution of the loop.
The results were striking:
std::array:
$ ./a.out result: 0 time: 99.554109
std::vector:
$ g++ arrVsVec.cpp -O3 $ ./a.out result: 0 time: 30.734491
Unraveling the Mystery
Upon examining the disassembly in Godbolt, it became evident that the performance discrepancy stemmed from memory page residency within the process address space. While the std::vector had already been allocated and zero-filled, the std::array was stored in the global .bss section of the executable and its pages had not been paged in.
Resolving the Issue
To resolve this discrepancy, it is necessary to bring the std::array pages into the address space. This can be achieved by adding the following line at the beginning of the main function:
std::fill_n(v.data(), n, 1); // included in <algorithm>
or alternatively, on Linux, using mlock():
mlock(v.data(), v.size() * sizeof(v[0]));
Conclusion
This benchmark highlights the importance of page residency in the performance of iteration. By ensuring that the pages of both std::array and std::vector are present in the address space, the iteration performance can be significantly improved.
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