Windows® Internals, Sixth Edition, Part 2

The cache manager is a set of kernel-mode functions and system threads that cooperate with the memory manager to provide data caching for all Windows file system drivers (both local and network). In this chapter, we’ll explain how the cache manager, including its key internal data structures and functions, works; how it is sized at system initialization time; how it interacts with other elements of the operating system; and how you can observe its activity through performance counters. We’ll also describe the five flags on the Windows CreateFile function that affect file caching.

Note

None of the cache manager’s internal functions are outlined in this chapter beyond the depth required to explain how the cache manager works. The programming interfaces to the cache manager are documented in the Windows Driver Kit (WDK). For more information about the WDK, see http://www.microsoft.com/whdc/devtools/wdk/default.mspx.

Key Features of the Cache Manager

The cache manager has several key features:

Supports all file system types (both local and network), thus removing the need for each file system to implement its own cache management code

Uses the memory manager to control which parts of which files are in physical memory (trading off demands for physical memory between user processes and the operating system)

Caches data on a virtual block basis (offsets within a file)—in contrast to many caching systems, which cache on a logical block basis (offsets within a disk volume)—allowing for intelligent read-ahead and high-speed access to the cache without involving file system drivers (This method of caching, called fast I/O, is described later in this chapter.)

Supports “hints” passed by applications at file open time (such as random versus sequential access, temporary file creation, and so on)

Supports recoverable file systems (for example, those that use transaction logging) to recover data after a system failure

Although we’ll talk more throughout this chapter about how these features are used in the cache manager, in this section we’ll introduce you to the concepts behind these features.

Single, Centralized System Cache

Some operating systems rely on each individual file system to cache data, a practice that results either in duplicated caching and memory management code in the operating system or in limitations on the kinds of data that can be cached. In contrast, Windows offers a centralized caching facility that caches all externally stored data, whether on local hard disks, floppy disks, network file servers, or CD-ROMs. Any data can be cached, whether it’s user data streams (the contents of a file and the ongoing read and write activity to that file) or file system metadata (such as directory and file headers). As you’ll discover in this chapter, the method Windows uses to access the cache depends on the type of data being cached.

The Memory Manager

One unusual aspect of the cache manager is that it never knows how much cached data is actually in physical memory. This statement might sound strange because the purpose of a cache is to keep a subset of frequently accessed data in physical memory as a way to improve I/O performance. The reason the cache manager doesn’t know how much data is in physical memory is that it accesses data by mapping views of files into system virtual address spaces, using standard section objects (file mapping objects in Windows API terminology). (Section objects are the basic primitive of the memory manager and are explained in detail in Chapter 10.) As addresses in these mapped views are accessed, the memory manager pages in blocks that aren’t in physical memory. And when memory demands dictate, the memory manager unmaps these pages out of the cache and, if the data has changed, pages the data back to the files.

By caching on the basis of a virtual address space using mapped files, the cache manager avoids generating read or write I/O request packets (IRPs) to access the data for files it’s caching. Instead, it simply copies data to or from the virtual addresses where the portion of the cached file is mapped and relies on the memory manager to fault in (or out) the data into (or out of) memory as needed. This process allows the memory manager to make global trade-offs on how much memory to give to the system cache versus how much to give to user processes. (The cache manager also initiates I/O, such as lazy writing, which is described later in this chapter; however, it calls the memory manager to write the pages.) Also, as you’ll learn in the next section, this design makes it possible for processes that open cached files to see the same data as do processes that are mapping the same files into their user address spaces.

Cache Coherency