Multithreaded Directory Looping in C#

Question

I am trying to loop through all files and folders and perform an action on all files that have a certain extension. This method works fine, but I would like to make it multithreaded because when done over tens of thousands of files, it is really slow and I would imaging using multithreading would speed things up. I am just unsure about how to use threading in this case.

doStuff reads properties (date modified, etc. from the files and inserts them into a sqlite database. I am starting a transaction before the scan method is called so that is optimized as much as it can be.

Answers that provide the theory on how to do it are just as good as full working code answers.

    private static string[] validTypes = { ".x", ".y", ".z", ".etc" };
    public static void scan(string rootDirectory)
    {
        try
        {

            foreach (string dir in Directory.GetDirectories(rootDirectory))
            {

                if (dir.ToLower().IndexOf("$recycle.bin") == -1)
                    scan(dir);
            }

            foreach (string file in Directory.GetFiles(rootDirectory))
            {

                if (!((IList<string>)validTypes).Contains(Path.GetExtension(file)))
                {
                    continue;
                }


                doStuff(file);
            }
        }
        catch (Exception)
        {
        }
    }

Answer 1

Assuming that doStuff is thread-safe, and that you don't need to wait for the entire scan to finish, you can call both doStuff and scan on the ThreadPool, like this:

string path = file;
ThreadPool.QueueUserWorkItem(delegate { doStuff(path); });

You need to make a separate local variable because the anonymous method would have capture the file variable itself, and would see changes to it throughout the loop. (In other words, if the ThreadPool only executed the task after the loop continued to the next file, it would process the wrong file)

However, reading your comment, the main issue here is disk IO, so I suspect that multithreading will not help much.

Note that Directory.GetFiles will perform slowly for directories with large numbers of files. (Since it needs to allocate an array to hold of the filenames)
If you're using .Net 4.0, you can make it faster by calling the EnumerateFiles method instead, which uses an iterator to return a IEnumerable<string> that enumerates the directory as you run your loop.
You can also avoid the recursive scan calls with either method by passing the SearchOption parameter, like this:

foreach (string file in Directory.EnumerateFiles(rootDirectory, "*", SearchOption.AllDirectories))

This will recursively scan all subdirectories, so you'll only need a single foreach loop.
Note that this will exacerbate the performance issues with GetFiles, so you may want to avoid this pre-.Net 4.0.

Answer 2

Using multithreading on IO operations is generally a bad call*. You may have multiple CPUs or a CPU with multiple cores; but generally, your hard disk cannot read or write to multiple files at the same time. This sort of thing typically needs to be serialized.

That said, it's a good practice to perform this kind of work on a thread that's separate from your UI thread. That way the UI remains responsive while your app is doing its heavy lifting.

_{*I am assuming your scan and doStuff methods are actually reading and/or writing data on the hard disk. If this is not the case, parallelizing this code might make sense after all.}

Answer 3

What exactly does doStuff and scan do? Unless they're very CPU intensive I would have thought that the disk access would be the bottleneck and that if anything making it multithreaded might be slower.

Answer 4

On a side note, there's no need to cast validTypes to IList<string> because arrays implement IEnumerable<T> in .net 3.5+.

Secondly, validTypes might be better implemented as a HashSet, giving you O(1) lookup instead of O(n) with Contains. That said, this probably won't impact the performance in this case because your application is IO bound, as pointed out by the other answers.

Answer 5

Thanks for everyone who responded. What I ended up going with was

        foreach (string file in Directory.EnumerateFiles(rootDirectory, "*", SearchOption.AllDirectories))
        {
            if (!((IList<string>)validTypes).Contains(Path.GetExtension(file)))
            {
                continue;
            }
            string path = file;
            ThreadPool.QueueUserWorkItem(delegate { doStuff(path); });
        }

This ran in about 2 minutes compared to the multiple hours that it was taking before. I think most of the lag was in the database, not the file IO.

Thanks so much everyone!