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16.1. Chapter Introduction: Memory Management

Most data structures are designed to store and access objects of uniform size. A typical example would be an integer stored in a list or a queue. Some applications require the ability to store variable-length records, such as a string of arbitrary length. One solution is to store in list or queue a bunch of pointers to strings, where each pointer is pointing to space of whatever size is necessary to old that string. This is fine for data structures stored in main memory. But if the collection of strings is meant to be stored on disk, then we might need to worry about how exactly these strings are stored. And even when stored in main memory, something has to figure out where there are available bytes to hold the string. In a language like C++ or Java, programmers can allocate space as necessary (either explicitly with new or implicitly with a variable declaration). Where does this space come from? This section discusses memory management techniques for the general problem of handling space requests of variable size.

The basic model for memory management is that we have a (large) block of contiguous memory locations, which we will call the memory pool. Periodically, memory requests are issued for some amount of space in the pool. A memory manager has the job of finding a contiguous block of locations of at least the requested size from somewhere within the memory pool. Honoring such a request is called memory allocation. The memory manager will typically return some piece of information that the requestor can hold on to so that later it can recover the data that were just stored by the memory manager. This piece of information is called a handle. At some point, space that has been requested might no longer be needed, and this space can be returned to the memory manager so that it can be reused. This is called a memory deallocation. We can define an ADT for a simple memory manager for storing variable length arrays of integers as follows.

// Memory Manager abstract class
public interface MemManager {
  // Store a record and return a handle to it
  public MemHandle insert(byte[] info);

  // Release the space associated with a record
  public void release(MemHandle h);

  // Get back a copy of a stored record
  public byte[] getRecord(MemHandle h);

The user of the MemManager ADT provides a pointer (in parameter info) to space that holds some message to be stored or retrieved. This is similar to the C++ basic file read/write methods. The fundamental idea is that the client gives messages to the memory manager for safe keeping. The memory manager returns a receipt for the message in the form of a MemHandle object. The client holds the MemHandle until it wishes to get the message back.

Method insert lets the client tell the memory manager the length and contents of the message to be stored. This ADT assumes that the memory manager will remember the length of the message associated with a given handle, thus method getRecord does not include a length parameter but instead returns the message actually stored. Method release allows the client to tell the memory manager to release the space that stores a given message.

When all inserts and releases follow a simple pattern, such as last requested, first released (stack order), or first requested, first released (queue order), memory management is fairly easy. We are concerned here with the general case where blocks of any size might be requested and released in any order. This is known as dynamic memory allocation. One example of dynamic memory allocation is managing free store for a compiler's runtime environment, such as the system-level new and delete operations in C++. Another example is managing main memory in a multitasking operating system. Here, a program might require a certain amount of space, and the memory manager must keep track of which programs are using which parts of the main memory. Yet another example is the file manager for a disk drive. When a disk file is created, expanded, or deleted, the file manager must allocate or deallocate disk space.

A block of memory or disk space managed in this way is sometimes referred to as a heap. The term "heap" is being used here in a different way than the heap data structure typically used to implement a priority queue. Here "heap" refers to the memory controlled by a dynamic memory management scheme.

In the rest of this chapter, we first study techniques for dynamic memory management. We then tackle the issue of what to do when no single block of memory in the memory pool is large enough to honor a given request.

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