Key Features of the Cache Manager (II)

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).

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). 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 pages data out of the cache and back to the files that are open in (mapped into) the cache. 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 tradeoffs on how much memory to give to the system cache versus how much to give to user processes. 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.

Source of Information : Windows 7 Resource Kit 2009 Microsoft Press

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