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POCould someone explain the behaviour of the garbage collector?
 

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  1. POCould someone explain the behaviour of the garbage collector?
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    <p>I am playing around with the garbage collector in C# (or rather the CLR?) trying to better understand memory management in C#.</p> <p>I made a small sample program that reads three larger files into a <code>byte[]</code> buffer. I wanted to see, if </p> <ul> <li>I actually need to to anything in order to handle memory efficient</li> <li>it has any impact when setting the <code>byte[]</code> to null after the end of the current iteration </li> <li>and finally if it would help when forcing a garbage collection via <code>GC.Collect()</code></li> </ul> <p>Disclaimer: I measured memory consumption with windows task manager and rounded it. I tried several times, but overall it remained about the same.</p> <p>Here is my simple sample program:</p> <pre><code>static void Main(string[] args) { Loop(); } private static void Loop() { var list = new List&lt;string&gt; { @"C:\Users\Public\Music\Sample Music\Amanda.wma", // Size: 4.75 MB @"C:\Users\Public\Music\Sample Music\Despertar.wma", // Size: 5.92 MB @"C:\Users\Public\Music\Sample Music\Distance.wma", // Size: 6.31 MB }; Console.WriteLine("before loop"); Console.ReadLine(); foreach (string pathname in list) { // ... code here ... Console.WriteLine("in loop"); Console.ReadLine(); } Console.WriteLine(GC.CollectionCount(1)); Console.WriteLine("end loop"); Console.ReadLine(); } </code></pre> <p>For each test, I only changed the contents of the <code>foreach</code> loop. Then I ran the program, at each <code>Console.ReadLine()</code> I stopped and checked the memory usage of the process in windows task manager. I took notes of the used memory and then continued the program with return (I know about breakpoints ;) ). Just after the end of the loop, I wrote <code>GC.CollectionCount(1)</code> to the console in order to see how often the GC jumped in if at all.</p> <p><br /></p> <h2>Results</h2> <p><hr /> <strong>Test 1:</strong></p> <pre><code>foreach ( ... ) { byte[] buffer = File.ReadAllBytes(pathname); Console.WriteLine ... } </code></pre> <p>Result (memory used):</p> <pre><code>before loop: 9.000 K 1. iteration: 13.000 K 2. iteration: 19.000 K 3. iteration: 25.000 K after loop: 25.000 K GC.CollectionCount(1): 2 </code></pre> <hr /> <p><strong>Test 2:</strong></p> <pre><code>foreach ( ... ) { byte[] buffer = File.ReadAllBytes(pathname); buffer = null; Console.WriteLine ... } </code></pre> <p>Result (memory used):</p> <pre><code>before loop: 9.000 K 1. iteration: 13.000 K 2. iteration: 14.000 K 3. iteration: 15.000 K after loop: 15.000 K GC.CollectionCount(1): 2 </code></pre> <hr /> <p><strong>Test 3:</strong></p> <pre><code>foreach ( ... ) { byte[] buffer = File.ReadAllBytes(pathname); buffer = null; GC.Collect(); Console.WriteLine ... } </code></pre> <p>Result (memory used):</p> <pre><code>before loop: 9.000 K 1. iteration: 8.500 K 2. iteration: 8.600 K 3. iteration: 8.600 K after loop: 8.600 K GC.CollectionCount(1): 3 </code></pre> <hr /> <p><br /></p> <h2>What I dont understand:</h2> <ul> <li>In Test 1, the memory increases with each iteration. Therefore I guess that the memory is NOT freed at the end of the loop. But the GC still says it collected 2 times (<code>GC.CollectionCount</code>). How so?</li> <li>In Test 2, it obviously helps that <code>buffer</code> is set to <code>null</code>. The memory is lower then in Test 2. But why does <code>GC.CollectionCount</code> output 2 and not 3? <em>And why is the memory usage not as low as in Test 3?</em></li> <li>Test 3 uses the least memory. I would say it is so because 1. the reference to the memory is removed (<code>buffer</code> is set to <code>null</code>) and therefore when the garbage collector is called via <code>GC.Collect()</code> it can free the memory. Seems pretty clear.</li> </ul> <p>If anyone with more experience could shed some light on some of the points above, it would really help me. Pretty interesting topic imho.</p>
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    1. USgevorg
    1. USMax
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      1. POCould someone explain the behaviour of the garbage collector?
      1. USRubens Farias
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    1. COMore or less duplicate of http://stackoverflow.com/questions/1090451/net-garbage-collector-basics.
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      1. POCould someone explain the behaviour of the garbage collector?
      1. USBinary Worrier
    2. COVery big concept, I suggest you read the links provided in answers to the duplicate question.
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      1. POCould someone explain the behaviour of the garbage collector?
      1. USBinary Worrier
    3. COPerhaps you might also want to try specifying the GCCollectionMode by using a different overload of GC.Collect(), as detailed in the msdn article on Induced Collections (http://msdn.microsoft.com/en-us/library/bb384155.aspx)
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      1. POCould someone explain the behaviour of the garbage collector?
      1. USDaniel Robinson
 

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