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Measuring the memory usage of your C++ program
In C++, we might implement dynamic lists using the vector template. The int-valued constructor of the vector template allocates at least enough memory to store the provided number of elements in a contiguous manner. How much memory does the following code use?
std::vector<uint8_t> v1(10);
std::vector<uint8_t> v2(1000000);
The naive answer is 1000010 bytes or slightly less than 1 MB, but if you think a bit about it, you quickly realize that 1000010 bytes might be a lower bound. Indeed, the vector might allocate more memory and there is unavoidably some overhead for the vector instance.
Thankfully, it is easy to measure it. I wrote a little C++ program to measure actual memory usage in terms of allocated pages attributed to the program. We find that we use far more memory (2x or 4x more) than a naive analysis might suggest.
| start of the program | after the first vector | at the end | |
|---|---|---|---|
| ARM-based macOS | 1.25 MB | 1.25 MB | 2.25 MB |
| Intel-based Linux | 1.94 MB | 1.94 MB | 4.35 MB |
Interestingly, reserving memory may not use any new memory as pointed by a reader (Martin Leitner-Ankerl). In my tests, adding the following two lines did not change memory usage:
std::vector<uint8_t> v3;
v3.reserve(1000000000);
Further reading: Measuring memory usage: virtual versus real memory
Tooling: Ivica Bogosavljevic recommends that Linux users try heaptrack to better understand memory usage. Aleksey Kladov prefers Bytehound.