OSNews > Thread > "Reasons it is slower" by Morph

Why glxgears Is Slower with Kernel Mode Setting, Why It Doesn't Matter

Linked by Rahul on Sat 14th Mar 2009 00:00 UTC

Fedora Project has been on the forefont of development and adoption of kernel mode setting to enhance the desktop linux experience by making fairly invasive infrastructure improvements that affect the interaction between Xorg and the Linux kernel. In the past, one of the common way to test Xorg performance has been to use glxgears. While that hasn't been a particular good way to do it ever, the switch to kernel mode setting for Intel drivers ahead of the Fedora 11 Beta release to be available shortly has exposed the fallacy of this. In short, don't use glxgears. There are better methods to assess performance.

1 25 Comment(s)

http://osne.ws/gb1

Thread beginning with comment 353204
To read all comments associated with this story, please click here.

Reasons it is slower

by Morph on Sun 15th Mar 2009 01:10 UTC

Member since:
2007-08-20

"glXSwapBuffers() - basically, how fast we can push render buffers into the card... is slower with DRI2"

Does anyone know exactly why it is slower? Because there is an extra copy involved when copying data from userspace to kernel space? Or because of the system call overhead?

Reply Permalink Bookmark Score: 2

RE: Reasons it is slower

by cb_osn on Sun 15th Mar 2009 04:21 in reply to "Reasons it is slower"

Member since:
2006-02-26

Does anyone know exactly why it is slower? Because there is an extra copy involved when copying data from userspace to kernel space?

Without digging through the code myself, this would be my assumption. As far as I know, X has always dealt with accelerated graphics by using overlays-- which causes its own set of problems. If DRI2 is handling composition of 3D graphics by copying the data out of the GPU buffers and back into system memory, this would account for the decrease in speed. If this is the case, I can only hope that it is a stopgap measure and that they eventually intend to handle all compositing in the graphics hardware. Of course, given the way network transparency works in X and how deeply embedded it is in the graphics model, I'm not sure how they can reconcile those features.

Reply Parent Bookmark Score: 2

RE[2]: Reasons it is slower

by siride on Sun 15th Mar 2009 14:47 in reply to "RE: Reasons it is slower"

Member since:
2006-01-02

Please stop blaming network transparency. It's only relevant for the wire protocol, and even then, on a local machine, pretty fast alternative mechanisms are used instead of, e.g., TCP sockets.

Reply Parent Bookmark Score: 3