iPhone Objective-C, malloc or NSMutableData?

Question

I'm need to use a volitile block of memory to constantly write and rewrite the data inside using multiple threads. The data will be rendered thread-safe using @synchronized if I utilize either malloc'd data or NSMutableData.

My question is what is more recommended for speed? Seeing I'm running recursivly calculated equations on the matrix of data I need to be able to allocate, retrieve, and set the data as quickly as possible.

I'm going to be doing my own research on the subject but I was wondering if anyone knew off-hand if the overhead of the Objective-C NSMutableData would introduce speed setbacks?

Answer 1

re: psychotik's suggestion: volatile is a keyword in C that basically tells the compiler to avoid optimizing usage of the symbol it's attached to. This is important for multithreaded code, or code that directly interfaces with hardware. However, it's not very useful for working with blocks of memory (from malloc() or NSData.) As psychotik said, it's for use with primitives such as an int or a pointer (i.e. the pointer itself, not the data it points to.) It's not going to make your data access any faster, and may in fact slow it down by defeating the compiler's optimization tricks.

For cross-thread synchronization, your fastest bet is, I think, an OSSpinLock if you don't need recursive access, or a pthread_mutex set up as recursive if you do. Keep in mind OSSpinLock is, as the name suggests, a spin lock, so certain usage patterns make it less efficient than a pthread_mutex, but it's also extremely close to the metal (it's based off the hardware's atomic get/set operations.)

If your data really is being accessed frequently enough that you're concerned with locking performance, you'll probably want to avoid NSData and just work with a block of memory from malloc()--but, without knowing more about what you're trying to accomplish or how frequently you're accessing the data, a solution does not readily present itself. Can you tell us more about your intent?