Comparisons between GC and two other memory management methods

Question

I just want to understand more about current popular garbage collection, malloc / free and counter.

From my understanding, GC is the most popular because it relieves the burden of managing memory manually from the developers and also it is more bullet proof. malloc / free is easy to make mistake and cause memory leaks.

From http://ocaml.org/learn/tutorials/garbage_collection.html:

Why would garbage collection be faster than explicit memory allocation as in C? It's often assumed that calling free costs nothing. In fact free is an expensive operation which involves navigating over the complex data structures used by the memory allocator. If your program calls free intermittently, then all of that code and data needs to be loaded into the cache, displacing your program code and data, each time you free a single memory allocation. A collection strategy which frees multiple memory areas in one go (such as either a pool allocator or a GC) pays this penalty only once for multiple allocations (thus the cost per allocation is much reduced).

Is it true that GC faster than malloc / free?

Also, what if the counter style memory management (objective-c is using it) joins the party?

I hope someone can summary the comparisons with deeper insights.

The problem with this kind of question is that "GC" encompasses a huge design space with many, many variants that make very different trade offs, even more than malloc/free. — user395760

Stephen C Stephen C · Accepted Answer · 2014-03-30T03:02:41

Is is true that GC faster than malloc / free?

It can be. It depends on the memory usage patterns. It also depends on how you measure "faster". (For example, are you measuring overall memory management efficiency, individual calls to malloc / free, or ... pause times.)

But conversely, malloc / free typically makes better use of memory than a modern copying GC ... provided that you don't run into heap fragmentation problems. And malloc / free "works" when the programming language doesn't provide enough information to allow a GC to distinguish heap pointers from other values.

Also, what if the counter style memory management (objective-c is using it) joins the party?

The overheads of reference counting make pointer assignment more expensive, and you have to somehow deal with reference cycles.

On the other hand, reference counting does offer a way to control memory management pauses ... which can be a significant issue for interactive games / apps. And memory usage is also better; see above.

FWIW, the points made in the source that you quoted are true. But it is not the whole picture.

The problem is that the whole picture is ... too complicated to be covered properly in a StackOverflow answer.

Comparisons between GC and two other memory management methods

2 Answers