In C , the most basic memory allocation code is just a call to the new operator:
If you benchmark this line of code, you might find that it almost entirely free on a per-byte basis for large values ofs. But that is because we are cheating: the call to the new operation “virtually” allocates the memory, but you may not yet have actual memory that you can use. As you access the memory buffer, the system may then decide to allocate the memory pages (often in blocks of (bytes). Thus the cost of memory allocation can be hidden. The great thing with a virtual allocation is that if you never access the memory, you may never pay a price for it.
If you actually want to measure the memory allocation in C , then you need to ask the system to give yous bytes of allocated and initialized memory. You can achieve the desired result in C by adding parentheses after the call to the new operator:
Then the operating system actually needs to allocate and initialize memory
(2. It may still cheat in the sense that it may recycle existing blocks of memories or otherwise delay allocation. And I expect that it might do so routinely if the value ofsis small. But it gets harder for the system to cheat assgrows larger.
What happens if you allocate hundreds of megabytes in such a manner? The answer depends on the size of the pages. By default, your system probably uses small (4kB) pages. Under Linux, you can enable “transparent huge pages” which dynamically switches to large pages when large blocks of memory are needed. Using larger pages means having to allocate and access fewer pages, so it tends to be cheaper.
In both instances, I get around a couple of gigabytes per second on a recent Linux system (Ubuntu****************************. ) running a conventional Skylake processor. For comparison, you can set memory to zero at tens gigabytes per second and my disk can feed data to the system at more than 2 GB / s. Thus, at least on the system I am currently using, memory allocation is not cheap.My code is available, I use GNU GCC 8.3.