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[Daniel van Vugt <daniel.van.vugt@xxxxxxxxxxxxx>]

Good-bad news!

I can reproduce this bug again today, after updating my vivid desktop
for the first time in two weeks. Seems some kernel/Mesa change has made
it re-appear. And the INTEL_DEBUG workaround no longer works. But
--nbuffers=3 does.

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https://bugs.launchpad.net/bugs/1377872

Title:
  Double-buffered compositing performance is sometimes very poor (30
  FPS) on intel

Status in Mesa:
  Confirmed
Status in Mir:
  Confirmed
Status in mesa package in Ubuntu:
  Confirmed

Bug description:
  Double-buffered compositing performance is sometimes artificially poor
  on some intel systems. When this happens the frame rate seen is halved
  - about 30 FPS. However at the same time, Mir is observed to use very
  little render time and both the CPU and GPU are still mostly idle.
  It's just the Intel DRM logic sometimes takes two frames (~32ms) to
  complete a single page flip.

  Two known workarounds avoid the issue:
    (a) Add glFinish() into the mesa DisplayBuffer code; or
    (b) env INTEL_DEBUG=sync for the Mir server binary.

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