Speed to create and read data

Question

I have some small questions about the speed to create and read data in C/C++:

=> If I need to fill data in a array of any type (think about a 2048*2048 array), using a loop and fill each cell is faster then loading it from a file? (excluding time spent to open and close the file).

=> If have data in a separate file and read it, it costs the same time to read it from the original file? (imagine that I need to fill an array, is better to have this array filled on the main program or I can read without loss from a external file? (excluding the time to open and close the file))

=> Memcpy still fast if I need to copy a lot of data ?

Answer 1

The file operations will be MANY MANY MANY Times slower than memory operations.

memcpy is up to the compiler, but yes, in general it will do it quicker or just the same as you could without resorting to assembly.

Answer 2

If I need to fill data in a array of any type (think about a 2048*2048 array), using a loop and fill each cell is faster then loading it from a file? (excluding time spent to open and close the file).

where does data for you to fill, when you not read from file ? But in general, read from file is extremely slow. when reading on main memory is nearly atomic, a same operation on file is slower than 1000x or more. In practice, always to prevent to read from file if not necessary.

Memcpy still fast if I need to copy alot of data ?

yes. often it's faster, depend on Compiler and your hardware. because memcpy use some special CPU instruction for example SIMD (single intruction - multiply data) for performance, and maybe your CPU doesn't have it. Compiler still have this function for comparable.

Answer 3

In-memory operations are many orders faster than FILE IO operations, but you might be able to utilise a half way house.

Memory Mapped files use OS technology to map the contents of the file directly to memory without you having to read and copy each byte. You can then read/write the memory as normal. It's the basis of virtual memory in many architectures and as such is highly optimised and performant.