Debugging a performance issue on ListBoxDragDropTarget (Silverlight Toolkit)?

Question

I have a complex project using SilverLight Toolkit's ListBoxDragDropTarget for drag-drop operations and it is maxing CPU. I tried to reproduce the issue in a small sample project, but then it works fine. The problem persists when I remove our custom styles and all other controls from the page, but the page is hosted in another page's ScrollView.

"EnableRedrawRegions" shows that the screen gets redrawn on every frame. My question is this: How can I track down the cause of this constant redrawing?

Answer 1

I have used XPerf to help track down performance issues related to redrawing in Silverlight. It is not completely straightforward or an easy process, but it can help point you in the right direction to where your problems are.

I started with a great tutorial by Seema about using the XPerf command-line tool to profile CPU usage for a Silverlight app. You can basically load up your app, start sampling with XPerf, perform your CPU intensive operations, and then stop sampling and analyze the profile XPerf generates. When you look at the XPerf charts you can select can filter by some process (such as iexplorer or your browser) to see the total % CPU. You can then select a specific length of time in the profile and drill down to see what functions from which DLLs are taking the most CPU cycles. If you point XPerf to Microsoft's symbol server you should get the specific names of the functions where the app is spending most of its time.

For a Silverlight app it's most important to look at what's going on in agcore.dll, npctrl.dll, and coreclr.dll. If your performance problems are related to redrawing, most of the CPU time is likely spent in agcore.dll since that does most of the graphics related work for Silverlight. You can then drill into that and see the specific functions in agcore.dll that are getting called most often during your sample time.

I understand it is kind of an annoying way to debug since you can only really see what is going on in the core Silverlight functions, but it may be able to help you figure out what is going on. In my case I was able to see that most of the time was spent calculating drop-shadows in agcore.dll. I was then able to figure out I stupidly had some content within a drop-shadow effect that was changing many times a second and causing constant recalculation/redraws of the entire drop-shadow effect.

Once you identify your redrawing issues you might want to look into GPU Acceleration with BitmapCaching if you haven't already. That will help offload some of the redrawing to the GPU and save you some CPU cycles.