Managing Memory with Multiple Heaps

IBM C and C++ Compilers lets you create and use your own pools of memory, called heaps. You can use your own heaps in place of or in addition to the default IBM C and C++ Compilers runtime heap to improve the performance of your program.

Note: Many readers will not be interested in creating their own heaps. Using your own heaps is entirely optional, and your applications will work perfectly well using the default memory management provided (and used by) the IBM C and C++ Compilers runtime library. If you want to improve the performance and memory management of your program, multiple heaps can help you. Otherwise, you can ignore this section and any heap-specific library functions.

Why Use Multiple Heaps?
Using a single runtime heap is fine for most programs. However, using multiple heaps can be more efficient and can help you improve your program's performance and reduce wasted memory for a number of reasons:

• When you allocate from a single heap, you may end up with memory blocks on different pages of memory. For example, you might have a linked list that allocates memory each time you add a node to the list. If you allocate memory for other data in between adding nodes, the memory blocks for the nodes could end up on many different pages. To access the data in the list, the system may have to swap many pages, which can significantly slow your program.

  With multiple heaps, you can specify which heap you allocate from. For example, you might create a heap specifically for the linked list. The list's memory blocks and the data they contain would remain close together on fewer pages, reducing the amount of swapping required.

• In multithread applications, only one thread can access the heap at a time to ensure memory is safely allocated and freed. For example, say thread 1 is allocating memory, and thread 2 has a call to free. Thread 2 must wait until thread 1 has finished its allocation before it can access the heap. Again, this can slow down performance, especially if your program does a lot of memory operations.

  If you create a separate heap for each thread, you can allocate from them concurrently, eliminating both the waiting period and the overhead required to serialize access to the heap.

• With a single heap, you must explicitly free each block that you allocate. If you have a linked list that allocates memory for each node, you have to traverse the entire list and free each block individually, which can take some time.

  If you create a separate heap for that linked list, you can destroy it with a single call and free all the memory at once.

• When you have only one heap, all components share it (including the IBM C and C++ Compilers runtime library, vendor libraries, and your own code). If one component corrupts the heap, another component might fail. You may have trouble discovering the cause of the problem and where the heap was damaged.

  With multiple heaps, you can create a separate heap for each component, so if one damages the heap (for example, by using a freed pointer), the others can continue unaffected. You also know where to look to correct the problem.

You can create heaps of regular memory or shared memory, and you can have any number of heaps of any type. See Types of Memory for more information about the different types of memory for heaps. The only limit is the space available on your operating system (your machine's memory and swapper size, minus the memory required by other running applications).

IBM C and C++ Compilers provides heap-specific versions of the memory management functions, for example, umalloc and so on. Debug versions of all memory management functions are provided, including the heap-specific ones. IBM C and C++ Compilers also provides additional functions that you can use to create and manage your own heaps of memory, such as udefault.

Memory Management Functions
Types of Memory
Debugging Memory Heaps

Creating and Using a Fixed Size Heap
Creating and Using an Expandable Heap
Debugging Problems with Heap Memory
Changing the Default Heap Used in a Program
Example of Creating and Using a User Heap
Example of Creating and Using a Shared-Memory User Heap