Does anyone know of a tool to display the memory layout of a c++ program?

Memory fragmentation is a problem that can occur in some long lived apps that delete and assign memory a lot. The problem is the developer can never know that he even has a problem unless there is a tool that displays the heap layout. It would need a graphic display of the entire heap and the "holes" in the memory that aren't getting reallocated.

On a PC or other modern workstation that uses virtual memory, those holes don't really mean much. They are only logical holes since the OS will page out lesser-used blocks of memory and page in blocks when needed.

If you start to come close to exhausting your memory space (e.g. 2GB or 3GB on a 32-bit program) and need another large allocation then those are important, but for most PC programmers it hasn't been an issue for two decades.

If you're on a console or embedded system, or a system that does not use virtual memory, then you will likely be using your own custom allocator. Inside your custom allocator you can build statistics on memory fragments.

If you're on a system where it is an actual potential problem, you'll need a custom allocator. Pull from one side for long-term allocations and operate on a stack-based system; allocate on entering that level of the stack, release on exiting that level of the stack, and make sure every system operates on a push/pop design. Short term allocations pull from the opposite side of memory and have strict rules about when you can and cannot allocate. Google can find lots of examples of the rules and requirements for such systems since it needs to match your system's design.

I think you need to analyse the memory usage first.

Then, based on the results of the memory fragmentation analysis you can decide whether you need to use a custom allocator or not. Does that make sense?

Its pretty easy to replace new and delete and #define malloc/realloc/free, then log all data and call the real malloc/free or some OS function. It can change how the memory is fragmented though.

Or you could map all memory using VirtualQuery, but that probably wont help you much as the C/C++ allocation functions can preallocate and keep freed memory.

Thanks Hodgman,

I'll have a look at that.

Incidentally, I made a suggestion to Apple to add memory fragmentation analysis to Instruments and they seemed to be receptive to the idea (iOS is the platform I develop for)

If you think it is going to be a major issue, then I would advise you to write your own memory manager and add a metrics system.

The way we work is we have multiple memory managers designed for different scenarios. From a very simple scratchpad, to a complex self defragging render pool, we have loads of them in game.

As you can imagine, this can get very confusing and complicated.

To deal with that we have a metrics system. It is very simple in principle. Every time you allocate or free memory, it creates a structure and sends it to a manager.

The manager has a ring buffer and whenever it is full it sends it over the network to an external app. The external app then matches allocations and frees and monitors the state of memory.

It is incredibly useful for finding memory leaks and keeping a very tight view of the memory use of a game, but it slows the game down a lot. So you have to use it with caution.

I don't think if you are a casual coder working on a hobby game it is worth while, but if you want to do a full AAA game, it's essential.

I wish! Lots of companies are still making the switch from x86 to 64bit even now My previous employer shipped their first 64bit PC game this year... And only because they were forced to by the address space limit. Regular 32bit x86 address space is really tight for modern games these days.