Is accessing statically or dynamically allocated memory faster?

Question

There are 2 ways of allocating global array in C:

1. statically

   char data[65536];
   

2. dynamically

   char *d         
   
   
           

Answer 1

I would say you really should profile it. Theoretically you are right but there are some basic things you have to remember.

Language C is a high-level language like many there exist today and you tell the machine what to do. Getting closer to machine code would be considering ASM or similar. If you build code, through compiling and linking or whatever, the compiler will try the best to correctly run what you demand and optimize it (unless you don't want that). Remember, there also exist concepts like Just-In-Time compilation (JIT).

So I consider it hard to answer your question. For one thing you can be sure. A static array will most likely be faster especially with the size of 65536 because there are more chances of optimization for the compiler. This might depend on what size you defined. For GCC 65536 bytes seems to be common for stacks and caches, not sure. Some compilers might even tell you the array is too big, because they try to keep it in other memory hierarchies like caches which also are faster than Random Access Memory.

Last but not least remember that modern operating systems also have their memory management using virtual memory.

Static memory can be stored in data segments and will most likely be loaded when the program is executed, but remember this is also time you have to consider. Allocate the memory by the OS when the program is started or do it at runtime? It really depends on your application.

So I think you really should benchmark your results and see by how much faster it is. But as tendency I would say your static array will compile a code that is going to run faster.