Will Garbage Collected C be Faster Than C++?

Question

I had been wondering for quite some time on how to manager memory in my next project. Which is writing a DSL in C/C++.

It can be done in any of the three ways.

1. Reference counted C or C++.
2. Garbage collected C.
3. In C++, copying class and structures from stack to stack and managing strings separately with some kind of GC.

The community probably already has a lot of experience on each of these methods. Which one will be faster? What are the pros and cons for each?

A related side question. Will malloc/free be slower than allocating a big chunk at the beginning of the program and running my own memory manager over it? .NET seems to do it. But I am confused why we can't count on OS to do this job better and faster than what we can do ourselves.

Answer 1

It all depends! That's a pretty open question. It needs an essay to answer it!

Hey.. here's one somebody prepared earlier:

http://lambda-the-ultimate.org/node/2552

http://www.hpl.hp.com/personal/Hans_Boehm/gc/issues.html

It depends how big your objects are, how many of them there are, how fast they're being allocated and discarded, how much time you want to invest optimizing and tweaking to make optimizations. If you know the limits of how much memory you need, for fast performance, I would think you can't really beat grabbing all the memory you need from the OS up front, and then managing it yourself.

The reason it can be slow allocating memory from the OS is that it deals with lots of processes and memory on disk and in ram, so to get memory it's got to decide if there is enough. Possibly, it might have to page another processes memory out from ram to disk so it can give you enough. There's lots going on. So managing it yourself (or with a GC collected heap) can be far quicker than going to the OS for each request. Also, the OS usually deals with bigger chunks of memory, so it might round up the size of requests you make meaning you could waste memory.

Have you got a real hard requirement for going super quick? A lot of DSL applications don't need raw performance. I'd suggest going with whatever's simplest to code. You could spend a lifetime writing memory management systems and worrying which is best.

Answer 2

Why would garbage collected C be faster than C++? The only garbage collectors available for C are pretty inefficient things, more designed to plug memory leaks than to actually improve the quality of your code.

In any case, C++ has the potential for reaching better performance with less code (note that it's only a potential. It's also very possible to write C++ code that is far slower than the equivalent C).

Considering the current state of both languages, GC's are not currently going to improve performance in your code. GC's can be made very efficient in languages designed for it. C/C++ are not among those. ;)

Apart from that, it's impossible to say. Languages don't have a speed. It doesn't make sense to ask which language is faster. It depends on 1) the specific code, 2) the compiler that compiles it, and 3) the system it's running on (hardware as well as OS).

malloc is a fairly slow operation, far slower than the .NET equivalents, so yes, if you are performing a lot of small allocations, you may be better off allocating a large pool of memory once, and then using chunks of that.

The reason is that the OS has to find a free chunk of memory, basically by following a linked list of all free memory areas. In .NET, a new() call is basically nothing more than moving the heap pointer as many bytes as required by the allocation.