CODEWITHSUNDEEP
c#.netmultithreading
Kaskorian
Kaskorian

Reputation: 446

Speed improvement using multiple threads

I have a CustomControl called PlaylistView. It displays elements in a playlist with name and thumbnail. The method DisplayPlaylist ensures that a thread is started, in which the individual elements are added one by one and the thumbnails (30th frame) are read out:

public void DisplayPlaylist(Playlist playlist)
{
    Thread thread = new Thread(() => DisplayElements(playlist));
    thread.Start();
}

private void DisplayElements(Playlist playlist)
{
    for (int i = 0; i < playlist.elements.Count; i++)
        DisplayElement(playlist.elements[i], i);
}

private void DisplayElement(IPlayable element, int index)
{
    VideoSelect videoSelect = null;

    if (element is Audio)
        //
    else if (element is Video)
        videoSelect = new VideoSelect(index, element.name, GetThumbnail(element.path, SystemData.thumbnailFrame));

    videoSelect.Location = GetElementsPosition(index);

    panel_List.BeginInvoke(new Action(() => 
    {
        panel_List.Controls.Add(videoSelect);
    }));
}

private Bitmap GetThumbnail(string path, int frame)
{
    VideoFileReader reader = new VideoFileReader();
    try
    {
        reader.Open(path);

        for (int i = 1; i < frame; i++)
            reader.ReadVideoFrame();

        return reader.ReadVideoFrame();
    }
    catch
    {
        return null;
    }
}

But there is a problem.

It is much too slow (about 10 elements/sec). With a playlist length of 614, you would have to wait more than a minute until all are displayed. Each time you change the playlist, such as adding or deleting an item, the procedure starts with the new item. Adding 2 or more will make it even more complicated.

I now had the approach to use multiple threads and the number of threads used for this to be specified by the user (1 to max 10). The implementation in the code currently looks like this (only changed parts compared to the previously posted code)

public void DisplayPlaylist(Playlist playlist)
{
    for (int i = 0; i < SystemData.usedDisplayingThreads; i++)
    {
        Thread thread = new Thread(() => DisplayElements(playlist, i));
        thread.Start();
    }
}

private void DisplayElements(Playlist playlist, int startIndex)
{
    for (int i = startIndex; i < playlist.elements.Count; i += SystemData.usedDisplayingThreads)
        DisplayElement(playlist.elements[i], i);
}

The problem is that now very often null is returned by the GetThumbnail function, so an error occurs. In addition, a System.AccessViolationException is often thrown out.

In my opinion, the reason for this is the presence of multiple, simultaneously active VideoFileReaders. However, I do not know what exactly triggers the problem so I cannot present any solution. Maybe you know what the actual trigger is and how to fix the problem or maybe you also know other methods for speed improvement, which maybe even more elegant.

Upvotes: 2

Views: 205

Answers (1)

dlxeon
dlxeon

Reputation: 2010

I would start with logging what exception is raised in GetThumbnail method. Your code hides it and returns null. Change to catch (Exception exc), write exception details in log or at least evaluate in debugger. That can give a hint.

Also I'm pretty sure your VideoFileReader instances are IDisposable, so you have to dispose them by invoking reader.Close(). Maybe previous instances were not disposed and you are trying to open same file multiple times.

Update: video frame has to be disposed as well. Probably you will need to do a copy of bitmap if it is referenced with reader and prevents disposion.

Upvotes: 1

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