Why leaking memory is slower than doing delete[] on the dynamic array

Question

I just started using google benchmark for microbenchmarking and I got results I can't really explain. I have a function URLify(basic functions that encodes whitespaces). I convert my string to char * and pass it to the function

This is how I test it using google benchmark and Full optimization. using VS 2015 x64

while (state.KeepRunning()) {

    char* ch = new char[str.length()*2 ]; //str is a string I want to encode
    memcpy(ch, &str[0], str.length() + 1);  
    URLify(ch, str.length());
    delete[] ch;
}

this is the results doing 30000 iterations and 5 repetitions

BenchURLify/iterations:30000/repeats:5                    5370 ns         5729 ns        30000
BenchURLify/iterations:30000/repeats:5                    5366 ns         5208 ns        30000
BenchURLify/iterations:30000/repeats:5                    5349 ns         5208 ns        30000
BenchURLify/iterations:30000/repeats:5                    5364 ns         5729 ns        30000
BenchURLify/iterations:30000/repeats:5                    5356 ns         5208 ns        30000
BenchURLify/iterations:30000/repeats:5_mean               5361 ns         5417 ns            5
BenchURLify/iterations:30000/repeats:5_median             5364 ns         5208 ns            5
BenchURLify/iterations:30000/repeats:5_stddev             8.48 ns          285 ns            5

But when I removed delete[] from the code google benchmark showed different result and not the result I expected. I thought freeing memory every iteration would be slower than leaking memory. But this is the results without delete[] ch

BenchURLify/iterations:30000/repeats:5                    7240 ns         7292 ns        30000
BenchURLify/iterations:30000/repeats:5                    7245 ns         7292 ns        30000
BenchURLify/iterations:30000/repeats:5                    7116 ns         7292 ns        30000
BenchURLify/iterations:30000/repeats:5                    7091 ns         7292 ns        30000
BenchURLify/iterations:30000/repeats:5                    7116 ns         6771 ns        30000
BenchURLify/iterations:30000/repeats:5_mean               7162 ns         7188 ns            5
BenchURLify/iterations:30000/repeats:5_median             7116 ns         7292 ns            5
BenchURLify/iterations:30000/repeats:5_stddev             74.6 ns          233 ns            5

So my question is why doing delete[] shows better performance than leaking memory? or what am I missing here

Answer 1

The delete[] function is doing very little and will be extremely fast. The system will likely keep returning the same memory on each iteration and can all be done in userspace ( what happens in the kernel during malloc? has a lot more details).

Memory is allocated to your process in blocks. If you let the memory leak you will at some points need to extend memory space using a kernel call. These kernel calls are likely to be much more expensive than the delete calls.