A memory-mapped file is a segment of virtual memory that has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on disk, but can also be a device, shared memory object, or other resource that the operating system can reference through a file descriptor. Once present, this correlation between the file and the memory space permits applications to treat the mapped portion as if it were primary memory.

Memory-mapped I/O uses the filesystem to establish a virtual memory mapping from user space directly to the applicable filesystem pages. With a memory-mapped file, we can pretend that the entire file is in memory and that we can access it by simply treating it as a very large array. Memory Mapped Files. File mapping is the association of a file's contents with a portion of the address space of a process. The system creates a file mapping to associate the file and the address space of the process. A mapped region is the portion of address space that the process uses to access the file's contents.

• 1History
• 3Types

Another drawback of memory-mapped files relates to a given architecture's address space: a file larger than the addressable space can have only portions mapped at a time, complicating reading it. For example, a 32-bit architecture such as Intel's IA-32 can only directly address 4. Sep 12, 2018  Sharing of files - memory mapped files are particularly convenient when the same data is shared between multiple processes. Lower memory pressure - when resources run tight, the kernel can easily drop data from the disk cache out of main memory and into backing disk storage (clean pages don't even need to be written back to disk).

History[edit]

TOPS-20 PMAP[edit]

An early day^[1] implementation of this was the PMAP system call on the DEC-20's TOPS-20 operating system,^[2] a feature used by Software House's System-1022 database system.^[3]

SunOS 4 mmap[edit]

SunOS 4^[4] introduced Unix's mmap, which permitted programs 'to map files into memory.'^[5]

Growable Memory-Mapped Files (GMMF)[edit]

Two decades after the release of TOPS-20's PMAP, Windows NT was given Growable Memory-Mapped Files (GMMF).

Since 'CreateFileMapping function requires a size to be passed to it' and alteringa file's size is not readily accommodated, a GMMF API was developed.^[6] Use of GMMF requires declaring the maximum to which the file size can grow, but no unused space is wasted.

Benefits[edit]

The benefit of memory mapping a file is increasing I/O performance, especially when used on large files. For small files, memory-mapped files can result in a waste of slack space^[7] as memory maps are always aligned to the page size, which is mostly 4 KiB. Therefore, a 5 KiB file will allocate 8 KiB and thus 3 KiB are wasted. Accessing memory mapped files is faster than using direct read and write operations for two reasons. Firstly, a system call is orders of magnitude slower than a simple change to a program's local memory. Secondly, in most operating systems the memory region mapped actually is the kernel's page cache (file cache), meaning that no copies need to be created in user space.

Certain application-level memory-mapped file operations also perform better than their physical file counterparts. Applications can access and update data in the file directly and in-place, as opposed to seeking from the start of the file or rewriting the entire edited contents to a temporary location. Since the memory-mapped file is handled internally in pages, linear file access (as seen, for example, in flat file data storage or configuration files) requires disk access only when a new page boundary is crossed, and can write larger sections of the file to disk in a single operation.

A possible benefit of memory-mapped files is a 'lazy loading', thus using small amounts of RAM even for a very large file. Trying to load the entire contents of a file that is significantly larger than the amount of memory available can cause severe thrashing as the operating system reads from disk into memory and simultaneously writes pages from memory back to disk. Memory-mapping may not only bypass the page file completely, but also allow smaller page-sized sections to be loaded as data is being edited, similarly to demand paging used for programs.

The memory mapping process is handled by the virtual memory manager, which is the same subsystem responsible for dealing with the page file. Memory mapped files are loaded into memory one entire page at a time. The page size is selected by the operating system for maximum performance. Amplifier software free download. Since page file management is one of the most critical elements of a virtual memory system, loading page sized sections of a file into physical memory is typically a very highly optimized system function.^[8]

Types[edit]

There are two types of memory-mapped files:

Persisted[edit]

Persisted files are associated with a source file on a disk. The data is saved to the source file on the disk once the last process is finished. These memory-mapped files are suitable for working with extremely large source files.^[9]

Non-persisted[edit]

Non-persisted files are not associated with a file on a disk. When the last process has finished working with the file, the data is lost. These files are suitable for creating shared memory for inter-process communications (IPC).^[9]

Drawbacks[edit]

The major reason to choose memory mapped file I/O is performance. Nevertheless, there can be tradeoffs. The standard I/O approach is costly due to system call overhead and memory copying. The memory-mapped approach has its cost in minor page faults—when a block of data is loaded in page cache, but is not yet mapped into the process's virtual memory space. In some circumstances, memory mapped file I/O can be substantially slower than standard file I/O.^[10]

Another drawback of memory-mapped files relates to a given architecture's address space: a file larger than the addressable space can have only portions mapped at a time, complicating reading it. For example, a 32-bit architecture such as Intel's IA-32 can only directly address 4 GiB or smaller portions of files. An even smaller amount of addressable space is available to individual programs—typically in the range of 2 to 3 GiB, depending on the operating system kernel.

I/O errors on the underlying file (e.g. its removable drive is unplugged or optical media is ejected, disk full when writing, etc.) while accessing its mapped memory are reported to the application as the SIGSEGV/SIGBUS signals on POSIX, and the EXECUTE_IN_PAGE_ERROR structured exception on Windows. All code accessing mapped memory must be prepared to handle these errors, which don't normally occur when accessing memory.

Only hardware architectures with an MMU can support memory-mapped files. On architectures without an MMU, the operating system can copy the entire file into memory when the request to map it is made, but this is extremely wasteful and slow if only a little bit of the file will be accessed, and can only work for files that will fit in available memory.

Common uses[edit]

Perhaps the most common use for a memory-mapped file is the process loader in most modern operating systems (including Microsoft Windows and Unix-like systems.) When a process is started, the operating system uses a memory mapped file to bring the executable file, along with any loadable modules, into memory for execution. Most memory-mapping systems use a technique called demand paging, where the file is loaded into physical memory in subsets (one page each), and only when that page is actually referenced.^[11] In the specific case of executable files, this permits the OS to selectively load only those portions of a process image that actually need to execute.

Another common use for memory-mapped files is to share memory between multiple processes. In modern protected mode operating systems, processes are generally not permitted to access memory space that is allocated for use by another process. (A program's attempt to do so causes invalid page faults or segmentation violations.) There are a number of techniques available to safely share memory, and memory-mapped file I/O is one of the most popular. Two or more applications can simultaneously map a single physical file into memory and access this memory. For example, the Microsoft Windows operating system provides a mechanism for applications to memory-map a shared segment of the system's page file itself and share data via this section.

Platform support[edit]

Most modern operating systems or runtime environments support some form of memory-mapped file access. The function mmap(),^[12] which creates a mapping of a file given a file descriptor, starting location in the file, and a length, is part of the POSIX specification, so the wide variety of POSIX-compliant systems, such as UNIX, Linux, Mac OS X^[13] or OpenVMS, support a common mechanism for memory mapping files. The Microsoft Windows operating systems also support a group of API functions for this purpose, such as CreateFileMapping().^[14]

Some free portable implementations of memory-mapped files for Microsoft Windows and POSIX-compliant platforms are:

• Boost.Interprocess,^[15] in Boost C++ Libraries
• Boost.Iostreams,^[16] also in Boost C++ Libraries
• Fmstream^[17]
• Cpp-mmf^[18]

The Java programming language provides classes and methods to access memory mapped files, such as FileChannel.

The D programming language supports memory mapped files in its standard library (std.mmfile module).^[19]

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Ruby has a gem (library) called Mmap, which implements memory-mapped file objects.

Since version 1.6, Python has included a mmap module in its Standard Library.^[20] Details of the module vary according to whether the host platform is Windows or Unix-like.

For Perl there are a several modules available for memory mapping files on the CPAN, such as Sys::Mmap^[21] and File::Map.^[22]

In the Microsoft .NET runtime, P/Invoke can be used to use memory mapped files directly through the Windows API. Managed access (P/Invoke not necessary) to memory mapped files was introduced in version 4 of the runtime (see Memory-Mapped Files). For previous versions, there are third-party libraries which provide managed API's.^[23]

PHP supported memory-mapping techniques in a number of native file access functions such as file_get_contents() but has removed this in 5.3 (see revision log).

C++ Memory Mapped File

For the R programming language there exists a library on CRAN called bigmemory which uses the Boost library and provides memory-mapped backed arrays directly in R. The package ff offers memory-mapped vectors, matrices, arrays and data frames.

The J programming language has supported memory mapped files since at least 2005. It includes support for boxed array data, and single datatype files. Support can be loaded from 'data/jmf' J's Jdb and JD database engines use memory mapped files for column stores.

References[edit]

1. ^Development began 1969, shipped 1976
2. ^'TOPS-20 Monitor Calls Reference Manual'(PDF).
3. ^'System 1022 Database System'. We had a PMAP cache for file I/O(like PA1050) in extended sections.
4. ^Dec. 1988
5. ^Chris Siebenmann (7 June 2018). 'The history of Unix's confusing set of low-level ways to allocate memory'.
6. ^Jeffrey Richter (October 1995). 'Add Growable Memory-Mapped Files to your App'. Microsoft Systems Journal. pp. 17–28.
7. ^http://www.devshed.com/c/a/BrainDump/Using-mmap-for-Advanced-File-IO/
8. ^, 'What Do Memory-Mapped Files Have to Offer?'.
9. ^ ^ab'Memory-Mapped Files'. Microsoft Developer Network. Retrieved 4 January 2016.
10. ^http://lists.freebsd.org/pipermail/freebsd-questions/2004-June/050371.html, read vs. mmap (or io vs. page faults) by Matthew Dillon
11. ^'Demand Paging'
12. ^Memory Mapped FilesArchived 9 February 2007 at the Wayback Machine
13. ^Apple - Mac OS X Leopard - Technology - UNIXArchived 23 April 2009 at the Wayback Machine
14. ^CreateFileMapping Function (Windows)
15. ^'Sharing memory between processes: Memory Mapped Files'. Boost.org.
16. ^'Memory-Mapped Files'. Boost.org.
17. ^'Memory Mapped Files for Windows and POSIX systems'. SourceForge.
18. ^'cpp-mmf'. GitHub.
19. ^'std.mmfile - D Programming Language'. Digital Mars. Retrieved 4 December 2011.
20. ^'New Modules in 1.6'. Archived from the original on 30 December 2006. Retrieved 23 December 2008.
21. ^'Sys::Mmap Perl Module'.
22. ^'File::Map Perl Module'.
23. ^DotNetArchived 19 April 2010 at the Wayback Machine

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. Starting with the .NET Framework 4, you can use managed code to access memory-mapped files in the same way that native Windows functions access memory-mapped files, as described in Managing Memory-Mapped Files.

There are two types of memory-mapped files:

• Persisted memory-mapped files

  Persisted files are memory-mapped files that are associated with a source file on a disk. When the last process has finished working with the file, the data is saved to the source file on the disk. These memory-mapped files are suitable for working with extremely large source files.

• Non-persisted memory-mapped files

  Non-persisted files are memory-mapped files that are not associated with a file on a disk. When the last process has finished working with the file, the data is lost and the file is reclaimed by garbage collection. These files are suitable for creating shared memory for inter-process communications (IPC).

Processes, Views, and Managing 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.

To work with a memory-mapped file, you must create a view of the entire memory-mapped file or a part of it. You can also create multiple views to the same part of the memory-mapped file, thereby creating concurrent memory. For two views to remain concurrent, they have to be created from the same memory-mapped file.

Multiple views may also be necessary if the file is greater than the size of the application’s logical memory space available for memory mapping (2 GB on a 32-bit computer).

There are two types of views: stream access view and random access view. Use stream access views for sequential access to a file; this is recommended for non-persisted files and IPC. Random access views are preferred for working with persisted files.

Memory-mapped files are accessed through the operating system’s memory manager, so the file is automatically partitioned into a number of pages and accessed as needed. You do not have to handle the memory management yourself.

The following illustration shows how multiple processes can have multiple and overlapping views to the same memory-mapped file at the same time.

The following image shows multiple and overlapped views to a memory-mapped file:

Programming with Memory-Mapped Files

The following table provides a guide for using memory-mapped file objects and their members.

Security

You can apply access rights when you create a memory-mapped file, by using the following methods that take a MemoryMappedFileAccess enumeration as a parameter:

Memory Mapped File Example Linux

You can specify access rights for opening an existing memory-mapped file by using the OpenExisting methods that take an MemoryMappedFileRights as a parameter.

In addition, you can include a MemoryMappedFileSecurity object that contains predefined access rules.

To apply new or changed access rules to a memory-mapped file, use the SetAccessControl method. To retrieve access or audit rules from an existing file, use the GetAccessControl method.

Examples

Persisted Memory-Mapped Files

The CreateFromFile methods create a memory-mapped file from an existing file on disk.

The following example creates a memory-mapped view of a part of an extremely large file and manipulates a portion of it.

The following example opens the same memory-mapped file for another process.

Non-Persisted Memory-Mapped Files

The CreateNew and CreateOrOpen methods create a memory-mapped file that is not mapped to an existing file on disk.

The following example consists of three separate processes (console applications) that write Boolean values to a memory-mapped file. The following sequence of actions occur:

1. Process A creates the memory-mapped file and writes a value to it.

2. Process B opens the memory-mapped file and writes a value to it.

3. Process C opens the memory-mapped file and writes a value to it.

4. Process A reads and displays the values from the memory-mapped file.

5. After Process A is finished with the memory-mapped file, the file is immediately reclaimed by garbage collection.

To run this example, do the following:

1. Compile the applications and open three Command Prompt windows.

2. In the first Command Prompt window, run Process A.

3. In the second Command Prompt window, run Process B.

4. Return to Process A and press ENTER.

5. In the third Command Prompt window, run Process C.

6. Return to Process A and press ENTER.

The output of Process A is as follows:

Process A

Memory Mapped File Python

Process B

Process C

Windows Memory Mapped File Example C++

See also