[luiceur]

I've noticed that running simpleFoam solver, I get the same ClockTime for different time steps. Can this be possible? If so, what is really ClockTime meassuring? When running on large number of MPI processes `time mpirun simpleFoam...` seconds are considerably higher than ClockTime seconds.

Time = 4

smoothSolver:  Solving for Ux, Initial residual = 0.171835, Final residual = 0.0091272, No Iterations 3
smoothSolver:  Solving for Uy, Initial residual = 0.159782, Final residual = 0.00892248, No Iterations 3
smoothSolver:  Solving for Uz, Initial residual = 0.159237, Final residual = 0.00896156, No Iterations 3
GAMG:  Solving for p, Initial residual = 0.0278822, Final residual = 0.00197359, No Iterations 4
time step continuity errors : sum local = 0.00247324, global = 0.000155812, cumulative = -6.13125e-05
smoothSolver:  Solving for epsilon, Initial residual = 0.0348493, Final residual = 0.00304838, No Iterations 2
bounding epsilon, min: -0.00543397 max: 294.829 average: 0.65962
smoothSolver:  Solving for k, Initial residual = 0.314159, Final residual = 0.0162037, No Iterations 3
ExecutionTime = 2.06 s  ClockTime = 3 s

Time = 5

smoothSolver:  Solving for Ux, Initial residual = 0.0662, Final residual = 0.00334477, No Iterations 3
smoothSolver:  Solving for Uy, Initial residual = 0.0893908, Final residual = 0.00463377, No Iterations 3
smoothSolver:  Solving for Uz, Initial residual = 0.0862215, Final residual = 0.00460723, No Iterations 3
GAMG:  Solving for p, Initial residual = 0.0538329, Final residual = 0.00473931, No Iterations 3
time step continuity errors : sum local = 0.00278208, global = 0.000127678, cumulative = 6.63659e-05
smoothSolver:  Solving for epsilon, Initial residual = 0.0350506, Final residual = 0.00343218, No Iterations 2
bounding epsilon, min: -0.0526577 max: 1092.16 average: 0.862589
smoothSolver:  Solving for k, Initial residual = 0.224788, Final residual = 0.0123172, No Iterations 3
ExecutionTime = 2.18 s  ClockTime = 3 s

[GerhardHolzinger]

Originally Posted by luiceur

I've noticed that running simpleFoam solver, I get the same ClockTime for different time steps. Can this be possible? If so, what is really ClockTime meassuring? When running on large number of MPI processes `time mpirun simpleFoam...` seconds are considerably higher than ClockTime seconds.

Time = 4

ExecutionTime = 2.06 s  ClockTime = 3 s

Time = 5

ExecutionTime = 2.18 s  ClockTime = 3 s

What you see is OpenFOAM rounding the clock time to integer values of seconds. Thus, 2.06 and 2.18 both round to 2.

Roughly explained, the execution time is the time that OpenFOAM is doing its thing, the clock time is the time that passed on a wall clock. The operating system will occasionally cause OpenFOAM to wait, e.g. for a hard drive to spin-up or seek files. Also, even if you are doing nothing apart from watch OpenFOAM crunch numbers, occasionally other processes take a bit of CPU time. The time in which the CPU does something different than run OpenFOAM adds to the clock time, but not to the execution time.