https://bugs.freedesktop.org/show_bug.cgi?id=83012
--- Comment #4 from Eero Tamminen eero.t.tamminen@intel.com --- (In reply to comment #3)
Why don't we disable vsync by default? I think that's what proprietary drivers do.
Because that's relevant only for benchmarks. Normal applications want to present user complete (non-tearing) frames.
Note: with DRI2, using fullscreen with vsync off causes X to do copy of every frame (which has clear performance penalty for memory bandwidth bound benchmarks). With DRI3, when Vsync is disabled on client side, the X side copy is done only every Vsync, but currently you would need quad buffering to avoid stalling from X server side synching, which isn't what Mesa does (see bug 79715).
(If you're very close, but not quite at 60 or 30 FPS, this non-Vsync induced copy can cause your FPS even to slightly decrease compared to Vsync.)