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POUnusual Garbage Collection Patterns
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  1. POUnusual Garbage Collection Patterns
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    copied!<p>I'm seeing some unusual garbage collection patterns under .net 4.0 that I can't explain, any advice would be appreciated. </p> <p>At various times in the running of my program the G2 collection count begins to increase and GC % time goes to near 100%. This continues for some time before stopping (please see Performance Monitor screen grab below). During this time a PerfView trace of the CLR GC ETW events shows that a number of long running G2 collections are being triggered back to back with reason 'AllocSmall'. </p> <p>Can anyone shed some light on:</p> <ol> <li>Preciesly what causes an 'AllocSmall' event</li> <li>Why these are directly triggering G2 collection</li> <li>Why these G2 collections are occuring back to back</li> </ol> <p>LowMemory does not appear to be the problem as the GC start events do not carry this as their reason code (<a href="http://msdn.microsoft.com/en-us/library/ff356162.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/ff356162.aspx</a>). We have also seen similar situations in which G0 collections occur back to back.</p> <p><strong>Edit:</strong> It's been suggested that the trigger could be some threshold being exceeded. Given there is still available memory I would expect the G2 heap to expand at this point rather than have the GC thrash trying to collect.</p> <p><strong>Edit:</strong> The pictures posted below are much higher resolution than StackOverflow displays - just open the image link directly in a browser tab.</p> <p><img src="https://i.stack.imgur.com/H1WYL.png" alt="Performance Monitor Trace"> <img src="https://i.stack.imgur.com/EcVDy.png" alt="PerfView Event Trace"></p>
 

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