I have been testing out several approaches for writing out to files and I thought that the results are interesting enough to share. In all cases, I was writing a 128Kb buffer of random data to a file with size of 256Mb.

The first thing that I wanted to try was the trivial managed memory map approach:

using (var mmf = MemoryMappedFile.CreateFromFile("test.bin", FileMode.Create, "test", 1024*1024*256))
{
    using (var accessor = mmf.CreateViewAccessor())
    {
        for (int i = 0; i < accessor.Capacity; i += buffer.Length)
        {
            accessor.WriteArray(i, buffer, 0, buffer.Length);
        }
        accessor.Flush();
    }
}

This completed in 3.871 seconds.

Next, I wanted to see what would happen if I were using direct memory access, and used CopyMemory to do that:

[DllImport("kernel32.dll", EntryPoint = "RtlMoveMemory")]
static extern void CopyMemory(byte* dst, byte* src, long size);

using (var mmf = MemoryMappedFile.CreateFromFile("test.bin", FileMode.Create, "test", 1024*1024*256))
{
    using (var accessor = mmf.CreateViewAccessor())
    {
        byte* p = null;
        accessor.SafeMemoryMappedViewHandle.AcquirePointer(ref p);
        fixed (byte* src = buffer)
        {
            for (int i = 0; i < accessor.Capacity; i += buffer.Length)
            {
                CopyMemory(p + i, src, buffer.Length);
            }
        }
        accessor.SafeMemoryMappedViewHandle.ReleasePointer();
        accessor.Flush();
    }
}

As you can see, this is somewhat more complex, and requires unsafe code. But, this completed in 2.062 seconds. Nearly twice as fast.

Then I decided to try with raw file IO:

using (var f = new FileStream("test.bin",FileMode.Create))
{
    f.SetLength(1024*1024*256);
    for (int i = 0; i < f.Length; i += buffer.Length)
    {
        f.Write(buffer, 0, buffer.Length);
    }
    f.Flush(true);
}

This is about the most trivial code that you can think of, and this completed in about 1.956 seconds. Slightly faster, but within the margin of error (note, in repeated tests, they were consistently very close, and the file I/O was very near).

So, in other words, the accessor code adds a lot of overhead when using Memory Mapped Files.