Vista Aero: What a CPU Hog!

Microsoft's new Aero Glass GUI - one of the cornerstones of the company's Windows Vista marketing message - is a joy to behold. Aero's sleek, semi-transparent facades serve to enhance the user experience by providng a stronger sense of "depth" and cohesion. Combined with Vista's enhanced UI metaphors (love those "breadcrumbs" in explorer), Aero is a major improvement over the XP GUI.

It's also a CPU hog. Despite Microsoft's claims about leveraging 3D accelerator technology to offload the GUI workload, Aero still chews-up more CPU cycles (an average of 22%) with desktop composition enabled (i.e. 3D accelerated mode) than with it disabled (i.e. non-accelerated "legacy" mode). In other words, turn on the "bling" and you toss nearly a quarter of your CPU bandwidth out the window.

Note: I measured the above using the recently updated DMS Clarity Tracker Agent, which is now part of the new public exo.performance.network (xpnet.com) project. You can reproduce the test scenario by downloading the companion DMS Clarity Studio tool (also at xpnet.com) and running the OfficeBench test script, first with composition enabled and then with it disabled.

Other interesting tidbits:

• The ratio of the increased CPU overhead is roughly 4:1 in favor of User vs.Privileged (i.e. kernel mode) time - a good thing.
  
  
• That User mode bump can be traced directly to the participating applications - for example, Word 2007 using 48% more CPU time with composition enabled.
  
  
• Enabling desktop composition also causes windows to chew another 16% of the available physical RAM as measured by the Committed Bytes counter.
  
  
• Though CPU overhead was higher with composition enabled, overall script completion times - as measured by OfficeBench - were only 3% slower.
  
  
• One alarming statistic: Processor Queue Length, a measure of how many threads are queued and waiting for CPU time, increases by 28% with composition enabled - not good, especially for multitasking.
  

Bottom Line: For single-tasking users running generic productivity application scenarios, the added overhead from enabling desktop composition shouldn't be much a factor. However, more demanding users - especially those needing maximum performance - may find that turning composition off let's them regain some of those long lost CPU cycles.

I guess the old saw still applies: What Intel (and AMD) giveth, Microsoft taketh away...