Add info about "vblank_mode=0@" when doing glxspheres test

[RasmusWL]

If users don't add "vblank_mode=0" when they run their glxspheres tests, they might be getting strange results, as the GPU will try to match the screens refresh rate.

When I first ran my test, I thought the new primus backend was only as good as the Intel card, but when adding the "vblank_mode=0" it really showed how much better than virtualgl it was! - great job guys!

This is mentioned on primus' github page. :)
https://github.com/amonakov/primus
As well as why you shouldn't read into it too much. Oddly, with glxgears I got higher numbers with optirun -c yuv glxgears than primus, though when I played games primus' performance was visually much better.

[amonakov]

I'll add that primus could be optimized better for disabled vsync, and VirtualGL does not synchronize frame transfer by default: try VGL_FPS=1 optirun glxgears ;)

The best benchmarks are the games you play, after all.

[RasmusWL]

I just meant that when you tell people to test the performance difference by running a benchmark like glxspheres, like on the ubuntu ppa, it would be nice to add the "vblank_mode=0" info. Else the point of the benchmark would be lost, as the FPS would be limited to the screens refresh rate (in some cases).

[ArchangeGabriel]

This instructions are like this since VGL is still the default. We might consider reworking the doc on those things when primus will be widely used or being the default.