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On 2014-11-17T03:43:35+00:00 Daniel van Vugt wrote:

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)

Reply at: https://bugs.launchpad.net/mir/+bug/1377872/comments/5

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On 2014-11-17T04:04:10+00:00 Daniel van Vugt wrote:

I wonder if this is just a race?

If the page flip actually completes faster than select() takes to start
up then that would explain it.

Reply at: https://bugs.launchpad.net/mir/+bug/1377872/comments/7


** Changed in: mesa
       Status: Unknown => Confirmed

** Changed in: mesa
   Importance: Unknown => Medium

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

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

Status in Mesa:
  Confirmed
Status in Mir:
  In Progress
Status in “mesa” package in Ubuntu:
  New

Bug description:
  Double-buffered compositing performance is poor.

  Fortunately we don't use double-buffering in our default compositor
  (mostly), and Ubuntu touch does not use the default compositor either.
  However, if you make the compositor double-buffered, then it quickly
  drops down to 30 FPS while not consuming any significant CPU or render
  time.

  Test case A:
  Convert your compositor to double buffering by the code change suggested in bug 1350725.

  Test case B (different bug?):
  Switch all clients to double-buffering using this branch:  lp:~vanvugt/mir/double
  and then start a nested server.

  Now start a bunch of clients, and you will find they slow down to 30
  FPS after only starting a few. This does not happen with the default
  triple buffered compositor.

  I've been ignoring this issue for a little while [1] thinking I had simply run out of power, although suspected that can't be right as this is a powerful quad-core i7 and it's slowing down with only 10 clients.
  [1] https://bugs.launchpad.net/mir/+bug/1350725/comments/1

  Now today test case B has revealed (via MIR_CLIENT_PERF_REPORT) that
  the slowdown happens without using any significant render time (less
  than 2 ms) and without using any significant CPU (less than 20% of one
  out of four cores).

  So the conclusion is our default compositor is probably holding
  buffers for a little too long somewhere. Because that's the only
  sensible reason I can think of for my system to bog down to 30 FPS
  without stressing the CPU or GPU. We've got poor parallelism in our
  code somewhere. And once that's solved, we'll be able to make further
  improvements such as resolving bug 1350725.

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