[Bug 86366] New: Slow i965 performance (30 FPS); batching logic causes skipped frames

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=86366

            Bug ID: 86366
           Summary: Slow i965 performance (30 FPS); batching logic causes
                    skipped frames
           Product: Mesa
           Version: 10.3
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Drivers/DRI/i965
          Assignee: idr at freedesktop.org
          Reporter: daniel.van.vugt at canonical.com
        QA Contact: intel-3d-bugs at lists.freedesktop.org

In the Mir server code (DRI output) we call:
  1. eglSwapBuffers()
  2. get the new front buffer
  3. schedule a page flip: drmModePageFlip()

This works well, however if I force it to wait for the page flip immediately:
  4. select() on the DRM fd and then drmHandleEvent()
then step 4 (under some rare but predictable rendering loads) takes 32ms to
complete.

I've now confirmed it is just the page flip event that takes almost two frames
to arrive. And there are two workarounds that seem to successfully kick the
driver into action:
  3.5. glFinish()
or
  0. env INTEL_DEBUG=sync
Using either of these workarounds, rendering completes in about 1ms and select
then returns the next page flip event (~16ms interval).

So it seems the intel batching logic is deferring rendering way too long, or
the page flip event delivery is being deferred. However the two workarounds
suggest the former.

Using:
Mesa 10.3.2-0ubuntu1 (Ubuntu 15.04 vivid)
Intel® HD Graphics 4600 (Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz)

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