Comment 0 for bug 1613996

I noticed that a numerical solver I develop runs much slower on 16.04.1 than on 14.04. See for example this output:

The counters (top part of each result section) show that the solver does the same on both variants. The timings (lower part, beginning with FrameworkTimeWriteOutputs) are execution time in seconds. Overall time is in the last row (WallClockTime). First column shows results on Ubuntu 14.04, second column shows time on 16.04.1.

<pre>
../../data/tests/CCMTest/Kirchhoff.d6p
                                     Reference New
  IntegratorErrorTestFails 1026 == 1026
  IntegratorFunctionEvals 32474 == 32474
  IntegratorLESSetup 3114 == 3114
  IntegratorLESSolve 32473 == 32473
  IntegratorSteps 25809 == 25809
  LESJacEvals 463 == 463
  LESRHSEvals 3241 == 3241
  LESSetups 3114 == 3114
  --
  FrameworkTimeWriteOutputs 0.00 ~~ 0.00
  IntegratorTimeFunctionEvals 4.96 <> 9.46
  IntegratorTimeLESSetup 0.38 ~~ 0.58
  IntegratorTimeLESSolve 0.36 ~~ 0.35
  LESTimeJacEvals 0.08 ~~ 0.08
  LESTimeRHSEvals 0.27 ~~ 0.46
  WallClockTime 6.13 <> 10.79

MoistField.d6o
RHField.d6o
../../data/tests/EN15026/Kirchhoff.d6p
                                     Reference New
  IntegratorErrorTestFails 2 == 2
  IntegratorFunctionEvals 17685 == 17685
  IntegratorLESSetup 903 == 903
  IntegratorLESSolve 17684 == 17684
  IntegratorSteps 17635 == 17635
  LESJacEvals 295 == 295
  LESRHSEvals 2065 == 2065
  LESSetups 903 == 903
  --
  FrameworkTimeWriteOutputs 0.03 ~~ 0.03
  IntegratorTimeFunctionEvals 31.04 <> 58.89
  IntegratorTimeLESSetup 2.47 ~~ 3.76
  IntegratorTimeLESSolve 3.05 ~~ 2.98
  LESTimeJacEvals 0.28 ~~ 0.28
  LESTimeRHSEvals 2.02 ~~ 3.30
  WallClockTime 40.39 <> 69.39
</pre>

Particularly affected is the physics part of the code (IntegratorTimeFunctionEvals), which does by far the most memory access and uses pow(), sqrt(), exp() functions.

The test code was compiled with GCC 4.8.4 on Ubuntu 14.04 and was run unmodified on 16.04 (after upgrade and on a second machine after a fresh install).

When the code is compiled with the new GCC 5.4 on Ubuntu 16.04, the execution times are approximately the same as with GCC 4.8.4 on Ubuntu 16.04. Therefore I would not think it is a GCC bug.

I have a test suite archive for download and test execution prepared:

http://bauklimatik-dresden.de/downloads/tmp/test_suite.tar.7z

Run the test suite on 14.04 and on 16.04 and observe the numbers in the "New" column, they will differ significantly for most test cases.

Can you confirm my observation? And if yes, does anyone know how to avoid this performance drop?