ANTS Memory Profiler Walkthrough - Part 1

Using ANTS Memory Profiler to track down a memory leak in a WinForms application

This walkthrough shows how to locate a memory leak using a sample application called QueryBee. QueryBee is a simple WinForms application for running queries against SQL Server databases. It has a database connection dialog and a query window to query the database. We know our application is leaking memory, because every time we open a query window and close it again, our memory usage increases.

When you open the profiler, you first see the Startup screen:

Figure 1. The ANTS Memory Profiler startup screen.

Here, there’s a list of your recent profiling sessions so you can re-run them easily. For this example, we’ll start a new session by clicking New profiling session.

The New profiling session screen is displayed:

Figure 2. It's easy to configure and start a new profiling session.

All we need to do is point it at QueryBee, choose our performance counters, and click Start profiling.

The profiler starts up QueryBee and begins collecting performance counter data:

Figure 3. Whilst profiling, ANTS Memory Profiler collects performance counter data. The profiler is telling us that it's profiling our application. There are also some useful instructions on this screen telling us to take and compare snapshots.

Taking and comparing memory snapshots is a key activity when looking for memory leaks, so our approach will be as follows:

1. Wait for QueryBee to open.
2. Take a first snapshot without using the application; this first snapshot will be used as a baseline.
3. Within QueryBee, perform the actions that we think cause the memory leak.
4. Take a second snapshot.
5. Examine the comparison that the profiler shows us after it has finished taking and analyzing the second snapshot.

So, QueryBee is open, sitting in our system tray.
At this point, we take a first snapshot, which we will use as a baseline for comparison with later snapshots. 

When we click the Take Memory Snapshot button, the memory profiler forces a full garbage collection and takes a snapshot of the heap memory it is using.

Figure 4. Results from our first snapshot – Summary screen.

Now, we go back to QueryBee and perform the tasks which we think cause the memory leak.
We open up QueryBee and connect to a database.

Figure 5. QueryBee – Database connection dialog.

Figure 6. QueryBee – The query window.

The query window opens up and we enter and execute a SQL query.
We obtain some results and close the query window.

Figure 7. QueryBee – The results are displayed in a grid.

We close the query form.At this point, the window is gone. We expect the memory usage to fall back to where it was in the first snapshot, but that is not the case.

Figure 8. Despite closing our query window, the memory usage has not fallen.

So what's happening here? We take a second snapshot and get the results.

Figure 9. The summary pane compares the results of the two snapshots.

A number of problems are highlighted by the summary screen.

• We can see a large memory increase between snapshots, which we noticed on the timeline (top left).
• The Large Object Heap appears to be fragmented, which could cause problems (top right).
• The Generation 2 heap accounts for a large proportion of memory usage - often indicating objects are being held onto for longer than necessary (bottom left).

We can choose to select one of the largest classes which are shown to us in the bottom right of the screen, but instead we switch to the class list to find out more. The class list gives us a fuller picture of what's in the snapshot.

We're interested in objects which have been created since the baseline snapshot, so we need to look at types which have more instances in the second snapshot. We therefore sort by Instance Diff in decreasing order.

Figure 10. The class list allows you to compare memory usage in both snapshots in more detail.

The String class has been placed at the top of the list, with over 300,000 new instances. We want to understand why there is such a large increase so load the Instance Categorizer for the String class by clicking the icon.

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