MemoryMappedFile

This type provides a performance advantage. It is found in the System.IO.MemoryMappedFiles namespace. It is an abstract data type.

With MemoryMappedFile, we put a file into memory. In some cases it allows for better memory management than using arrays. MemoryMappedFile improves performance of binary file loading.

A simple start

Let's try just loading a file into a memory mapping. Make sure you create the "test.file" and add some characters to it. Then we load it into a mapping.

using System;
using System.IO.MemoryMappedFiles;

class Program
{
    static void Main()
    {
        using (MemoryMappedFile file =
            MemoryMappedFile.CreateFromFile("c:\\programs\\test.file"))
        using (MemoryMappedViewStream stream = file.CreateViewStream())
        {
            while (true)
            {
                // Read in byte from the MemoryMappedFile.
                int result = stream.ReadByte();
                // Zero bytes are past the end of the file.
                if (result == 0)
                {
                    break;
                }
                // Print file data to the console.
                Console.WriteLine("NUMBER: " + result);
                char letter = (char)result;
                Console.WriteLine("LETTER: " + letter);
            }
        }
    }
}
HELLO
NUMBER: 72
LETTER: H
NUMBER: 69
LETTER: E
NUMBER: 76
LETTER: L
NUMBER: 76
LETTER: L
NUMBER: 79
LETTER: O

Benchmark

This program loads a binary file of 4.37 MB that contains binary data. The file is essentially a grouping of about 1400 smaller files.

using System;
using System.Diagnostics;
using System.IO;
using System.IO.MemoryMappedFiles;

class Program
{
    static void Main()
    {
        const int max = 1;
        var s1 = Stopwatch.StartNew();
        for (int i = 0; i < max; i++)
        {
            Test1();
        }
        s1.Stop();
        var s2 = Stopwatch.StartNew();
        for (int i = 0; i < max; i++)
        {
            Test2();
        }
        s2.Stop();
        var s3 = Stopwatch.StartNew();
        for (int i = 0; i < max; i++)
        {
            Test3();
        }
        s3.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000 * 1000) /
            max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000 * 1000) /
            max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s3.Elapsed.TotalMilliseconds * 1000 * 1000) /
            max).ToString("0.00 ns"));
    }

    static void Test1()
    {
        // FileStream.
        using (FileStream file = File.Open("C:\\P.bin", FileMode.Open))
        {
            Read(file);
        }
    }

    static void Test2()
    {
        // MemoryMappedFile.
        using (MemoryMappedFile file = MemoryMappedFile.CreateFromFile("C:\\P.bin"))
        using (MemoryMappedViewStream stream = file.CreateViewStream())
        {
            Read(stream);
        }
    }

    static void Test3()
    {
        // MemoryStream.
        using (MemoryStream stream = new MemoryStream(File.ReadAllBytes("C:\\P.bin")))
        {
            Read(stream);
        }
    }

    static void Read(Stream stream)
    {
        // This method reads in the file-format specific values.
        using (BinaryReader reader = new BinaryReader(stream))
        {
            int count = reader.ReadInt32();
            for (int i = 0; i < count; i++)
            {
                string u = reader.ReadString();
                int len = reader.ReadInt32();
                byte[] b = reader.ReadBytes(len);
            }
        }
    }
}
9671400.00 ns
6737300.00 ns
6958400.00 ns
7246513.00 ns
4726050.00 ns
7294708.00 ns

Notes, benchmark

MemoryMappedFile is fast. For loading the file only once, MemoryMappedFile is faster than the other two approaches. The FileStream approach is the slowest.

A summary

MemoryMappedFile provides a way to load a file with good performance. It seems to have better performance than FileStream and also the File.ReadAllBytes approach.