Managing Memory

Windows NT can address 4 gigabytes of memory. Each application running on Windows NT has most of the lower 2 gigabytes of its virtual address space at its disposal. For a console application, the system uses 5.5 MB of the lower 2 gigabytes to permit you to view portions of the system that reside elsewhere. For a Windows application, the system uses to 9 MB for that purpose.

If your application is being ported from another operating system or from an earlier version of Windows, you might have developed a special virtual memory scheme for your own private use. Having another memory manager for your application decreases performance. To increase performance, let Windows NT manage the virtual address space.

Consider using file mapping in the following situations.

• You are randomly accessing a read-only file or a file that is only written to by one process. Shared writing to memory-mapped files from multiple processes requires a bit of internal system synchronization and does not work well if the file is remote, because you will have to manage the remote synchronization. Using memory-mapped files for sequential file access is faster than standard sequential file access, but uses more memory than the file system cache does. If you are going to access a file sequentially, use the FILE_FLAG_SEQUENTIAL_SCAN flag when calling the CreateFile function.
• You are using a temporary file and you know its maximum size. You can map a large temporary space which is backed by the system paging files instead of by an existing file. Simply pass 0xffffffff as the file handle to the CreateFileMapping function and specify the size you need. You can also call the VirtualAlloc function to create a large data space backed by the paging file, but this memory is not sharable with another process. However, do not create a large area backed by disk if your application must run on machines with limited disk space. Instead, use VirtualAlloc to reserve that amount of linear address space, then commit pages as you need them.
• You are managing communication between multiple processes. Memory-mapped files are faster than other mechanisms, such as named pipes, RPC, or shared file access. You might need a mutex to protect access to the shared section, but mutex operations are inexpensive.