WPF, Terminal Services & Bitmap Remoting

Everybody who has been playing around with WPF for a reasonable period of time might have known that WPF Application Processing is actually split up into two threads, they are UI thread which runs the dispatcher loop, and processes any GUI, keyboard/mouse/ink input, animation, layout and user initiated messages/events, and there is a hidden unmanaged background thread which is responsible for composing the graphical content (both in hardware or software if fallback is needed), and presenting it to frame buffer for display.

The whole architecture of WPF is built around this two threads model, and it has some of the benefits as articulated by Chris Anderson in the two-year long Channel video. One of the benefits Chris mentioned is that with the two threads model, in terminal services scenario, we can have the UI thread running in the TS server, and have the composition thread (or render thread in other nomenclature) running at the TS client console. And the communication between the two threads are handed over to the TS/RDP protocol. This can enable one interesting scenario called primitive remoting or vector remoting. And since UI thread is running at the TS server side, and it maintains the visual tree and the composition is at the TS client side, so in order to keep the client screen up to date, the UI thread will send the edits to the composition tree over the TS/RDP protocol in a highly compressed manner, this not only saves the network bandwidth, because only the edits need to be remoted, but also speeds up the client processing. This type of server/client communication also holds true in the standalone WPF application, the difference is that those two threads are running at the same Win32 process, and the inter-thread communication mechanism is used instead of the TS/RDP wire. To enable primitive remoting, both the TS server and TS client should run under Windows Vista and with desktop composition is enabled, this requirement tremendously narrows down the scenario in which the primitive remoting could be leveraged.

The upcoming .NET Framework 3.5 SP1 release would change all of this, in particular, bitmap remoting with sub region update support will always be used even if the TS server and the TS client are equipped to support primitive remoting, primitive remoting can be as good or bad as bitmap remoting, and the scenario in which primitive remoting is enabled is quite rare, because most existing Windows servers such as Windows 2000 or Windows 2003 server families don't support primitive remoting. So for any developer who wants your WPF applications to run reasonably well under TS scenario and running under TS is an important scenario for your applications, you need to take implication of bitmap remoting into consideration beforehand, there are a couple of ways from which you can improve the performance of your WPF application in TS scenario:

• Considering using as little animations as possible in your application, and if animations are indispensable, you could try reducing the DesiredFrameRate (The default value is 6O FPS) of each animations you need to use, usually 25~30 FPS should be enough, and if you don't need high fidelity animation experience, you can use a even lower frame rate.
• Considering using solid color brushes instead of gradient or tile brushes.
• Try reducing the number of "hidden redraws" your application needs to perform, "hidden redraws" will be overlaid by its most foreground redraws, but they will still be remoted which waste network bandwidth. You can visualize the "hidden redraws" using the Perforator (part of WPF Performance Suite) with "Show dirty-regions update overlay" CheckBox checked.