How The Garbage Collector Can Cause Random Slowness

In this article, I am going to use a number of tools to show how, because .NET applications run on top of the CLR, things are not always as straight forward as they seem. I’m also going to prove to you that high CPU usage and slow performance in your applications are not always caused by slow code, but sometimes by poor or excessive memory use.

The example that I will use is of an ASP.NET website where users have complained of random slowness.  As usual, the DBA has been blamed, but has investigated and assured everyone that the database isn't responsible, so it’s time to grab a profiler and dig into the problem.

I should point out that this article is a walkthrough of how to use different tools to track down performance issues, rather than a stick to beat the garbage collector with.  In this case the performance issues can be frustrating, but the garbage collector is doing its best with what the ASP.NET developers have given it.  The behaviour we’ll be seeing is also limited to ASP.NET, as it uses the “server-mode” garbage collection, which aims to optimise throughput over responsiveness.

The Problem

The demo site (downloadable from the top of this article) is made up of one page with three buttons; the first button uses large chunks of memory, the second frees that memory, and the third performs an operation which normally takes around 1 millisecond to complete.  When any of the buttons are pushed, the time of day and time to run are displayed. To make it easier to reproduce a problem, I am using a 32-bit process, so we are limited to a mere 2GB of memory (yes, I know I could push it to 3GB, but let’s not complicate matters).  In case you are wondering how realistic this demo is, with respect to having one button to immediately allocate loads of memory, in a real life situation there may well be a background operation that suddenly uses up great chunks of memory, or it might be lots of users hitting pages all at once which causes the memory usage to abruptly go up.

This exciting screenshot shows how button 1 usually takes ages to complete and, before it suddenly failed with an OutOfMemory Exception at 12:49:38, the longest button 3 had taken to complete its operation was 21 milliseconds. After this inexplicable failure, we start to see memory pressure, and button 3 can randomly take anything from 0 to 561 milliseconds to complete, so there is clearly a lack of consistency.

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