Tuesday, December 14, 2021

Ruminating on Memory Mapped Files

 Today, both Java and .NET have full support for memory mapped files. Memory mapped files are not only very fast for read/write operations, but they also serve another important function --- they can be shared between different processes. This enables us to use memory mapped files as shared data store for IPC. 

The following excellent articles by Microsoft and MathWorks explains how to use memory mapped files. 



Snippets from the above articles:

A memory-mapped file contains the contents of a file in virtual memory. This mapping between a file and memory space enables an application, including multiple processes, to modify the file by reading and writing directly to the memory. 

Memory-mapped files can be shared across multiple processes. Processes can map to the same memory-mapped file by using a common name that is assigned by the process that created the file.

Accessing files via memory map is faster than using I/O functions such as fread and fwrite. Data is read and written using the virtual memory capabilities that are built in to the operating system rather than having to allocate, copy into, and then deallocate data buffers owned by the process. 

Memory-mapping works best with binary files, and in the following scenarios:

  • For large files that you want to access randomly one or more times
  • For small files that you want to read into memory once and access frequently
  • For data that you want to share between applications
  • When you want to work with data in a file as if it were an array

Due to limits set by the operating system, the maximum amount of data you can map with a single instance of a memory map is 2 gigabytes on 32-bit systems, and 256 terabytes on 64-bit systems.

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