Memory Mapped File Quirks [Greg]

Memory mapped files are segments of virtual memory that are directly mapped to a physical file on disk, byte-by-byte. This technology has a number of benefits over traditional stream based I/O, such as performance during random access to large files, or the ability to share the mapped memory between different threads and processes. The .NET framework provides built-in support for memory mapped files starting from version 4. In most cases using memory mapped files in .NET is as easy as using traditional files streams: just map a file into the memory, open a seekable memory stream and use it as if it was a normal file while enjoying all the performance benefits. However, there are some subtle, but important differences. One of them is that memory mapped file views must always be aligned at the boundaries of the system’s virtual memory pages. An important consequence is that:

Compared to the size of the underlying file on disk, the size of a memory mapped view stream is always rounded up towards a multiple of the system’s virtual memory page size. You must take care to differentiate between useful data and junk bytes used to fill up the space.

A little background

A memory mapped file is a continuous chunk of memory in a process address space that has a direct byte-by-byte mapping to an external resource. Typically, such resource is a data file on the physical hard drive. However, it is also possible to represent device memory or any other OS object that can be accessed via a file handle as a memory mapped file structure. In Windows, memory mapped files can also be backed by the system page file rather than by a separate file system object.

In Windows, managing memory mapped files has been supported for a while (see the function reference). The .NET framework natively supports this technology starting from version 4.0 through the types in the System.IO.MemoryMappedFiles namespace.

In .NET you can create a memory mapping either for an existing file on the physical hard drive by specifying its path, or for an existing mapping by referring to the map name. Alternatively, you can create a new memory mapped file and have the system automatically manage a temporary file on disk to back it up. The temp file will be automatically deleted when you dispose of the memory mapped file; this is particularly useful when using memory mapped files for inter-process communication rather than for data persistence. In any case you need to use one of the static methods in the MemoryMappedFile class to create a memory map instance.

Read more: BCL Team Blog

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