Introduction to the Memory Manager

By default, the virtual size of a process on 32-bit Windows is 2 GB. If the image is marked specifically as large address space aware, and the system is booted with a special option (described later in this chapter), a 32-bit process can grow to be 3 GB on 32-bit Windows and to 4 GB on 64-bit Windows. The process virtual address space size on 64-bit Windows is 7,152 GB on IA64 systems and 8,192 GB on x64 systems. (This value could be increased in future releases.)

As you saw in Chapter 2, “System Architecture,” in Part 1 (specifically in Table 2-2), the maximum amount of physical memory currently supported by Windows ranges from 2 GB to 2,048 GB, depending on which version and edition of Windows you are running. Because the virtual address space might be larger or smaller than the physical memory on the machine, the memory manager has two primary tasks:

In addition to providing virtual memory management, the memory manager provides a core set of services on which the various Windows environment subsystems are built. These services include memory mapped files (internally called section objects), copy-on-write memory, and support for applications using large, sparse address spaces. In addition, the memory manager provides a way for a process to allocate and use larger amounts of physical memory than can be mapped into the process virtual address space at one time (for example, on 32-bit systems with more than 3 GB of physical memory). This is explained in the section Address Windowing Extensions later in this chapter.

Note

There is a Control Panel applet that provides control over the size, number, and locations of the paging files, and its nomenclature suggests that “virtual memory” is the same thing as the paging file. This is not the case. The paging file is only one aspect of virtual memory. In fact, even if you run with no page file at all, Windows will still be using virtual memory. This distinction is explained in more detail later in this chapter.

The memory manager is part of the Windows executive and therefore exists in the file Ntoskrnl.exe. No parts of the memory manager exist in the HAL. The memory manager consists of the following components:

Each of these components is covered in more detail later in the chapter.

Like all other components of the Windows executive, the memory manager is fully reentrant and supports simultaneous execution on multiprocessor systems—that is, it allows two threads to acquire resources in such a way that they don’t corrupt each other’s data. To accomplish the goal of being fully reentrant, the memory manager uses several different internal synchronization mechanisms, such as spinlocks, to control access to its own internal data structures. (Synchronization objects are discussed in Chapter 3, “System Mechanisms,” in Part 1.)

Some of the systemwide resources to which the memory manager must synchronize access include:

Per-process memory management data structures that require synchronization include the working set lock (held while changes are being made to the working set list) and the address space lock (held whenever the address space is being changed). Both these locks are implemented using pushlocks.

The Memory and Process performance counter objects provide access to most of the details about system and process memory utilization. Throughout the chapter, we’ll include references to specific performance counters that contain information related to the component being described. We’ve included relevant examples and experiments throughout the chapter. One word of caution, however: different utilities use varying and sometimes inconsistent or confusing names when displaying memory information. The following experiment illustrates this point. (We’ll explain the terms used in this example in subsequent sections.)