How to dynamically expand a Memory Mapped File

Question

I've used C# to solve the following requirement.. - create an app the can receive a lot of data fast - you must be able to analyse the received data while more are incoming. - use as little CPU and disk as possible

My idea for an algorithm was..

SIZE = 10MB
Create a mmf with the size of SIZE
On data recived:
  if data can't fit mmf: increase mmf.size by SIZE
  write the data to mmf

-> The size on the disc are increased in chuncks of 10MB when the previous "room/space" are used.

How is the "increase mmf.size by SIZE" done in C#? I have found a lot of simple examples on creating mmfs and views but the only place (link) I have seen code that acutally increases the mmfs area uses code that can't compile. Any help will be greatly appriciated.

EDIT This causes an exception :

private void IncreaseFileSize()
{
    int theNewMax = this.currentMax + INCREMENT_SIZE;
    this.currentMax = theNewMax;

    this.mmf.Dispose();

    this.mmf = MemoryMappedFile.CreateFromFile(this.FileName, FileMode.Create, "MyMMF", theNewMax);
    this.view = mmf.CreateViewAccessor(0, theNewMax);            
}

This exception is thrown : The process cannot access the file 'C:\Users\moberg\Documents\data.bin' because it is being used by another process.

Answer 1

The reason that the code does not compile is because it uses a non-existing overload. Either create a filestream yourself and pass it to the correct overload (assuming 2000 will be your new size):

FileStream fs = new FileStream("C:\MyFile.dat", FileMode.Open);
MemoryMappedFile mmf = MemoryMappedFile.CreateFromFile(fs, "someName", 2000,
 MemoryMappedFileAccess.ReadWriteExecute, null, HandleInheritablity.None, false);

Or use this overload to skip the filstream creation:

MemoryMappedFile mmf = MemoryMappedFile.CreateFromFile("C:\MyFile.dat", 
          FileMode.Open, "someName", 2000);

Answer 2

Use the overload of MemoryMappedFile.CreateFromFile that takes a capacity parameter.

Answer 3

Well, you can!!.

Here is my implementation of a growable memory mapped file:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.IO;
using System.IO.MemoryMappedFiles;

namespace MmbpTree
{
    public unsafe sealed class GrowableMemoryMappedFile : IDisposable
    {

        private const int AllocationGranularity = 64 * 1024;

        private class MemoryMappedArea
        {
            public MemoryMappedFile Mmf;
            public byte* Address;
            public long Size;
        }


        private FileStream fs;

        private List<MemoryMappedArea> areas = new List<MemoryMappedArea>();
        private long[] offsets;
        private byte*[] addresses;

        public long Length
        {
            get {
                CheckDisposed();
                return fs.Length;
            }
        }

        public GrowableMemoryMappedFile(string filePath, long initialFileSize)
        {
            if (initialFileSize <= 0 || initialFileSize % AllocationGranularity != 0)
            {
                throw new ArgumentException("The initial file size must be a multiple of 64Kb and grater than zero");
            }
            bool existingFile = File.Exists(filePath);
            fs = new FileStream(filePath, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.None);
            if (existingFile)
            {
                if (fs.Length <=  0 || fs.Length % AllocationGranularity != 0)
                {
                    throw new ArgumentException("Invalid file. Its lenght must be a multiple of 64Kb and greater than zero");
                }
            }
            else
            { 
                fs.SetLength(initialFileSize);
            }
            CreateFirstArea();
        }

        private void CreateFirstArea()
        {
            var mmf = MemoryMappedFile.CreateFromFile(fs, null, fs.Length, MemoryMappedFileAccess.ReadWrite,  null, HandleInheritability.None, true);
            var address = Win32FileMapping.MapViewOfFileEx(mmf.SafeMemoryMappedFileHandle.DangerousGetHandle(), 
                Win32FileMapping.FileMapAccess.Read | Win32FileMapping.FileMapAccess.Write,
                0, 0, new UIntPtr((ulong) fs.Length), null);
            if (address == null) throw new Win32Exception();

            var area = new MemoryMappedArea
            {
                Address = address,
                Mmf = mmf,
                Size = fs.Length
            };
            areas.Add(area);

            addresses = new byte*[] { address };
            offsets = new long[] { 0 };

        }


        public void Grow(long bytesToGrow)
        {
            CheckDisposed();
            if (bytesToGrow <= 0 || bytesToGrow % AllocationGranularity != 0)  {
                throw new ArgumentException("The growth must be a multiple of 64Kb and greater than zero");
            }
            long offset = fs.Length;
            fs.SetLength(fs.Length + bytesToGrow);
            var mmf = MemoryMappedFile.CreateFromFile(fs, null, fs.Length, MemoryMappedFileAccess.ReadWrite, null, HandleInheritability.None, true);
            uint* offsetPointer = (uint*)&offset;
            var lastArea = areas[areas.Count - 1];
            byte* desiredAddress = lastArea.Address + lastArea.Size;
            var address = Win32FileMapping.MapViewOfFileEx(mmf.SafeMemoryMappedFileHandle.DangerousGetHandle(), 
                Win32FileMapping.FileMapAccess.Read | Win32FileMapping.FileMapAccess.Write,
                offsetPointer[1], offsetPointer[0], new UIntPtr((ulong)bytesToGrow), desiredAddress);
            if (address == null) {
                address = Win32FileMapping.MapViewOfFileEx(mmf.SafeMemoryMappedFileHandle.DangerousGetHandle(),
                   Win32FileMapping.FileMapAccess.Read | Win32FileMapping.FileMapAccess.Write,
                   offsetPointer[1], offsetPointer[0], new UIntPtr((ulong)bytesToGrow), null);
            }
            if (address == null) throw new Win32Exception();
            var area = new MemoryMappedArea {
                Address = address,
                Mmf = mmf,
                Size = bytesToGrow
            };
            areas.Add(area);
            if (desiredAddress != address) {
                offsets = offsets.Add(offset);
                addresses = addresses.Add(address);
            }
        }

        public byte* GetPointer(long offset)
        {
            CheckDisposed();
            int i = offsets.Length;
            if (i <= 128) // linear search is more efficient for small arrays. Experiments show 140 as the cutpoint on x64 and 100 on x86.
            {
                while (--i > 0 && offsets[i] > offset);
            }
            else // binary search is more efficient for large arrays
            {
                i = Array.BinarySearch<long>(offsets, offset);
                if (i < 0) i = ~i - 1;
            }
            return addresses[i] + offset - offsets[i];
        }

        private bool isDisposed;

        public void Dispose()
        {
            if (isDisposed) return;
            isDisposed = true;
            foreach (var a in this.areas)
            {
                Win32FileMapping.UnmapViewOfFile(a.Address);
                a.Mmf.Dispose();
            }
            fs.Dispose();
            areas.Clear();
        }

        private void CheckDisposed()
        {
            if (isDisposed) throw new ObjectDisposedException(this.GetType().Name);
        }

        public void Flush()
        {
            CheckDisposed();
            foreach (var area in areas)
            {
                if (!Win32FileMapping.FlushViewOfFile(area.Address, new IntPtr(area.Size))) {
                    throw new Win32Exception();
                }
            }
            fs.Flush(true);
        }
    }
}

Here is the Win32FileMapping class:

using System;
using System.Runtime.InteropServices;

namespace MmbpTree
{
    public static unsafe class Win32FileMapping
    {
        [Flags]
        public enum FileMapAccess : uint
        {
            Copy = 0x01,
            Write = 0x02,
            Read = 0x04,
            AllAccess = 0x08,
            Execute = 0x20,
        }

        [DllImport("kernel32.dll", SetLastError = true)]
        public static extern byte* MapViewOfFileEx(IntPtr mappingHandle,
                                            FileMapAccess access,
                                            uint offsetHigh,
                                            uint offsetLow,
                                            UIntPtr bytesToMap,
                                            byte* desiredAddress);

        [DllImport("kernel32.dll", SetLastError = true)]
        public static extern bool UnmapViewOfFile(byte* address);


        [DllImport("kernel32.dll", SetLastError = true)]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool FlushViewOfFile(byte* address, IntPtr bytesToFlush);
    }
}

And here you have the Extensions class:

using System;

namespace MmbpTree
{
    public static class Extensions
    {
        public static T[] Add<T>(this T[] array, T element)
        {
            var result = new T[array.Length + 1];
            Array.Copy(array, result, array.Length);
            result[array.Length] = element;
            return result;
        }

        public static unsafe byte*[] Add(this byte*[] array, byte* element)
        {
            var result = new byte*[array.Length + 1];
            Array.Copy(array, result, array.Length);
            result[array.Length] = element;
            return result;
        }
    }
}

As you can see I take the unsafe approach. It's the only way to get the performance benefits of memory mapped files.

To work with this you need to consider the following concepts:

• The block or page. This is your minimal region of continuous memory address and storage space you work with. The size of a block or page must be a multiple of underlying system page size (4Kb).
• The initial file size. It must be a multiple of the block or page size and it must be a multiple of the system allocation granularity (64Kb).
• The file growth. It must be a multiple of the block or page size and it must be a multiple of the system allocation granularity (64Kb).

For example you may want to work with a page size of 1Mb, a file growth of 64Mb and an initial size of 1Gb. You can get a pointer to a page by calling GetPointer, grow the file using Grow and flush the file using Flush:

const int InitialSize = 1024 * 1024 * 1024;
const int FileGrowth = 64 * 1024 * 1024;
const int PageSize = 1024 * 1024;
using (var gmmf = new GrowableMemoryMappedFile("mmf.bin", InitialSize))
{
    var pageNumber = 32;
    var pointer = gmmf.GetPointer(pageNumber * PageSize);

    // you can read the page content:
    byte firstPageByte = pointer[0];
    byte lastPageByte = pointer[PageSize - 1];

    // or write it
    pointer[0] = 3;
    pointer[PageSize -1] = 43;


    /* allocate more pages when needed */
    gmmf.Grow(FileGrowth);

    /* use new allocated pages */

    /* flushing the file writes to the underlying file */ 
    gmmf.Flush();

}

Answer 4

Once you map a file in memory, you cannot increase its size. This is a known limitation of memory mapped files.

...you must calculate or estimate the size of the finished file because file mapping objects are static in size; once created, their size cannot be increased or decreased.

One strategy would be to use chunks stored in non-persisted memory mapped files of a given size, say 1GB or 2GB. You would manage these through a top level ViewAccessor of your own design (probably doing basic passthru of the methods you need from the MemoryMappedViewAccessor).

Edit: or you could just create a non-persisted memory mapped file of a maximal size you expect to use (say 8GB to start, with a parameter to tune it on start-up of your application) and retrieve MemoryMappedViewAccessor's per logical chunk. The non-persisted file will not use physical resources until each view is requested.