Trivial-MMAP

Trivial-MMAP is a Common Lisp library that aims to provide a high-level, easy-to-use API for working with memory-mapped files.

The library exports the following symbols:

• mmap-file a function that maps a file into memory and returns an object containing a pointer to the memory-mapped file along with other meta data;
• munmap-file a function that removes a memory-mapped file from memory;
• mmap-read-char a helper function that facilitates reading a character from a memory-mapped file;
• mmap-read-byte a helper function for reading one byte from a memory-mapped file;
• with-mmap-file a helper macro that ensures that a memory-mapped file is safely unmapped (using munmap-file function) after we are done with it.

In additions, the following are the exported symbols of the predicate and accessor functions for dealing with the memory-mapped file object returned by the mmap-file function:

• mmapped-file-p a predicate function that returns true if a variable is a memory-mapped file object (and false otherwise);
• mmapped-file-pointer an accessor function that, given a memory-mapped file object, returns a raw pointer address of the file;
• mmapped-file-size an accessor function that, given a memory-mapped file object, returns the size of the file;
• mmapped-file-offset an accessor function that, given a memory-mapped file object, returns the offset into the file.

Trivial-MMAP is based on code snippets in a blog post by Nicolas Martyanoff.

Getting Started

The following code snippet shows how to use Trivial-MMAP's with-mmap-file and mmap-read-char to read one character at a time from a file containing Twitter social graph (780 MB uncompressed).

CL-USER> (time
          (trivial-mmap:with-mmap-file (mmapped-file "/home/zodmaner/twitter_rv_15066953.net")
                    (loop while (< (trivial-mmap:mmapped-file-offset mmapped-file)
                                   (trivial-mmap:mmapped-file-size mmapped-file)) do
                       (assert (characterp (trivial-mmap:mmap-read-char mmapped-file))))))
Evaluation took:
  3.400 seconds of real time
  3.400000 seconds of total run time (3.360000 user, 0.040000 system)
  100.00% CPU
  8,479,536,815 processor cycles
  65,952 bytes consed

The little toy benchmark above was run on my laptop (an i5 ThinkPad X230 (Ivy Bridge) with 8 GB of RAM and an SSD). For comparison, reading the same file using with-open-file and read-char takes around 14 seconds of real time on the same machine.

Of course, you could avoid the with-mmap-file macro if you want to. The following code snippet is equivalent to the above, but only use the functions provided by Trivial-MMAP:

(time
 (let ((mmapped-file (trivial-mmap:mmap-file "/home/zodmaner/twitter_rv_15066953.net")))
  (unwind-protect
       (loop while (< (trivial-mmap:mmapped-file-offset mmapped-file)
                      (trivial-mmap:mmapped-file-size mmapped-file)) do
         (assert (characterp (trivial-mmap:mmap-read-char mmapped-file))))
    (trivial-mmap:munmap-file mmapped-file))))

Although from my experiences the version that uses the with-mmap-file macro is usually around 10% to 15% faster compared to the version that did not use the macro.

Dependencies

• Alexandria
• Osicat

Limitations

Of course, this library is far from being completed. Here is the list of some of the current limitations/missing features:

• Currently the library only supports reading data from memory-mapped files.
• The library only supports Linux. It may work on MacOS, but definitely not on Windows.

As always, patches and bug reports are more then welcome!

Author & Maintainer

Smith Dhumbumroong (zodmaner@gmail.com)

License

Public Domain