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    <p>I would note that most answers haven't read the example the code. This is not about spawning a bunch of threads and writing to disk, this is about spawning a bunch of threads, doing some work <em>new JavaScriptSerializer().Serialize(saeed);</em> and <strong>then</strong> writing to disk! </p> <p>This is important to note, because the longer that work takes the more benefit simple threading provides by making sure the disk isn't idling while computation takes place.</p> <hr> <p>The long and short of it is because you wrote some simplistic code, as others have explained:</p> <ol> <li>You are creating 90,000 threads - this is expensive and unnecessary! </li> <li>You are locking all the work, making this single threaded! <ol> <li>Yes without the lock you get an exception... that doesn't magically make the lock a good idea from a performance idea - it just means you've got erroneous code.</li> </ol></li> </ol> <p>A quick and easy way to get into threading - that's slightly less dangerous (though you can still stuff it up) is to use the Task Parallel Library. For example:</p> <pre><code>using System; using System.Diagnostics; using System.IO; using System.Threading.Tasks; namespace ConsoleApplication15 { class Program { const int FILE_COUNT = 9000; const int DATA_LENGTH = 100; static void Main(string[] args) { if (Directory.Exists(@"c:\Temp\")) Directory.Delete(@"c:\Temp\", true); Directory.CreateDirectory(@"c:\Temp\"); var watch = Stopwatch.StartNew(); for (int i = 0; i &lt; FILE_COUNT; i++) { string data = new string(i.ToString()[0], DATA_LENGTH); File.AppendAllText(string.Format(@"c:\Temp\{0}.txt", i), data); } watch.Stop(); Console.WriteLine("Wrote 90,000 files single-threaded in {0}ms", watch.ElapsedMilliseconds); Directory.Delete(@"c:\Temp\", true); Directory.CreateDirectory(@"c:\Temp\"); watch = Stopwatch.StartNew(); Parallel.For(0, FILE_COUNT, i =&gt; { string data = new string(i.ToString()[0], DATA_LENGTH); File.AppendAllText(string.Format(@"c:\Temp\{0}.txt", i), data); }); watch.Stop(); Console.WriteLine("Wrote 90,000 files multi-threaded in {0}ms", watch.ElapsedMilliseconds); } } } </code></pre> <p>The single threaded version runs in about 8.1 seconds, and the multi-threaded version runs in about 3.8 seconds. Note that my test values are different than yours.</p> <p>While the TPL's default settings aren't always optimised for the scenario you're working on, they provide a much better basis than running 90,000 threads! You'll also note that in this case I don't have to do any locking nor do I have to handle the closure - because the API presented already handles that for me.</p>
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    1. POWhy does threading increase time (reduce performance) in this example?
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    1. COHow will TPL tasks (which use the threadpool anyway) help here?
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      1. USSimon Whitehead
    2. COi updated my question
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    3. CO@SimonWhitehead TPL provides multithreading, which in my test did improve performance. TPL also provides a bunch of built in features, like thread-pooling, that significantly improve performance over the OP's attempts.
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