Cygwin/X causes NVIDIA GPU to turn on in Optimus systems

["Aaron A. Kelley" <aaronkelley@hotmail.com>]

For those that aren’t aware, NVIDIA Optimus is a feature that makes use of
the Intel GPU that is included in new Intel Core i series processors (Sandy
Bridge and up).  Basically, in a system that has both the integrated Intel
GPU and also a discrete NVIDIA GPU, the Intel GPU is used for most desktop
operations and the NVIDIA GPU kicks in when the graphics power is needed for
games and other processing jobs.  This is done to save power --- if the
NVIDIA GPU can spend most of its time powered down, your battery lasts
longer.  The NVIDIA card can be used to run only particular applications
individually, and its output is dumped over to the Intel GPU for display
using some fast framebuffer magic.

I usually leave Cygwin/X open on my machine because I use it to access GUI
apps running in SSH terminals from time to time.  Today I noticed that in an
Optimus environment, it is causing the NVIDIA GPU to kick in, which causes
the NVIDIA GPU to stay powered on when maybe it doesn’t need to.

http://stuff.aaron-kelley.net/2013/03/cygxoptimus.png

I went to the NVIDIA control panel and set xwin.exe specifically to use the
integrated GPU but it still says that xwin.exe is running on the NVIDIA GPU
when I run it.  Cygwin/X must be doing something unusual at initialization
that makes the NVIDIA card feel like it has to kick on.  I don’t have to
actually launch any X windows to see this occur, it happens moments after I
launch the Cygwin/X server.

I haven’t been able to find anyone else mentioning this.  It may be because
Optimus configurations aren’t very common.  This also means that this issue
may be tricky to track down.  I’m not sure if it will be easy to find
someone who knows about Cygwin/X development who also has access to an
Optimus environment.  This might be something that has to go for NVIDIA for
them to fix in their drivers, but it’d be nice to know a little more about
the issue before trying to get in touch with them.

I’m interested in doing anything I can do help address this but I don’t know
where to begin, so I’m welcoming your feedback.

Thanks,

-- 
Aaron


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