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parallel run is slower than serial run (pimpleFoam) !!! 

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August 13, 2013, 14:01 
parallel run is slower than serial run (pimpleFoam) !!!

#1 
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Hi all
I have run flow around a cylinder and plate. I have test my problem in serial and parallel execution. but the serial run (1 proc) is more faster than parallel run (2 proc) ? how can I improve my parallel run ? Best Regards 

August 13, 2013, 14:16 

#2 
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Kent Wardle
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You have left out some important info. http://www.cfdonline.com/Forums/ope...gethelp.html
How big is your problem size? Perhaps it is too small to benefit from parallel executionif you are running a 2d problem with 5000 cells I would not be surprised to see little or no speedup. What kind of machine are you running on? How did you execute the solver? (should be "mpirun np 2 solverName parallel" for 2 procs on local machine) 

August 13, 2013, 15:36 

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Quote:
Hi my problem have 180000 cells and it is a 2d problem I have run the bellow command mpirun np 2 pimpleFoam parallel I have run on my laptop with intel cpu core 2 due 

August 30, 2013, 10:27 

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any help will be appreciated
this is the decomposePar result : HTML Code:
Build : 1.6ext22f6e2e40a1e Exec : decomposePar force Date : Aug 30 2013 Time : 17:39:29 Host : yasVGNFW370J PID : 2983 Case : /home/yas/OpenFOAM/yas2.1.1/run/icoFsiFoamPiezo_RUNS/OKMESHcyl_PiezoPLTioFsiFoamPiezoThesisOF16ext/parallel nProcs : 1 SigFpe : Enabling floating point exception trapping (FOAM_SIGFPE). // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // Create time Time = 0 Removing 2 existing processor directories Create mesh for region region0 Calculating distribution of cells Selecting decompositionMethod scotch Finished decomposition in 0.47 s Calculating original mesh data Distributing cells to processors Distributing faces to processors Calculating processor boundary addressing Distributing points to processors Constructing processor meshes Processor 0 Number of cells = 92368 Number of faces shared with processor 1 = 503 Number of processor patches = 1 Number of processor faces = 503 Number of boundary faces = 185681 Processor 1 Number of cells = 91392 Number of faces shared with processor 0 = 503 Number of processor patches = 1 Number of processor faces = 503 Number of boundary faces = 183769 Number of processor faces = 503 Max number of processor patches = 1 Max number of faces between processors = 503 Processor 0: field transfer Processor 1: field transfer End. HTML Code:
Courant Number mean: 0 max: 0.00165125113506 velocity magnitude: 7.125 deltaT = 1.19904076739e06 Time = 1.19904076739e06 DILUPBiCG: Solving for Ux, Initial residual = 1, Final residual = 5.2997904141e08, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 1, Final residual = 3.92819713926e07, No Iterations 1 GAMG: Solving for p, Initial residual = 1, Final residual = 9.74149677961e06, No Iterations 292 GAMG: Solving for p, Initial residual = 0.00492661344204, Final residual = 9.92739792382e06, No Iterations 62 GAMG: Solving for p, Initial residual = 0.000438924562465, Final residual = 9.31995819834e06, No Iterations 20 time step continuity errors : sum local = 1.08325394028e08, global = 1.2131344075e10, cumulative = 1.2131344075e10 GAMG: Solving for p, Initial residual = 0.000895746588392, Final residual = 9.54211427674e06, No Iterations 20 GAMG: Solving for p, Initial residual = 0.000113652119668, Final residual = 9.24182927039e06, No Iterations 7 GAMG: Solving for p, Initial residual = 3.1040845323e05, Final residual = 7.79188809601e06, No Iterations 3 time step continuity errors : sum local = 9.06826897434e09, global = 3.05845168269e10, cumulative = 4.27158609019e10 DILUPBiCG: Solving for omega, Initial residual = 0.00276574153542, Final residual = 1.17965829568e06, No Iterations 2 DILUPBiCG: Solving for k, Initial residual = 0.999999997864, Final residual = 2.04240307473e06, No Iterations 3 DILUPBiCG: Solving for Ux, Initial residual = 0.00328278643814, Final residual = 3.83577769127e08, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 0.0166301820008, Final residual = 3.20367439059e07, No Iterations 1 GAMG: Solving for p, Initial residual = 0.000108129763017, Final residual = 8.78361949654e06, No Iterations 16 GAMG: Solving for p, Initial residual = 7.66538334551e05, Final residual = 8.24183510606e06, No Iterations 5 GAMG: Solving for p, Initial residual = 2.28286003354e05, Final residual = 6.74306412704e06, No Iterations 3 time step continuity errors : sum local = 7.86321574349e09, global = 2.93093380319e10, cumulative = 7.20251989338e10 GAMG: Solving for p, Initial residual = 1.17775650304e05, Final residual = 7.26056758009e06, No Iterations 1 GAMG: Solving for p, Initial residual = 7.52125937243e06, Final residual = 5.52916300329e06, No Iterations 1 GAMG: Solving for p, Initial residual = 5.88957082068e06, Final residual = 4.3767204652e06, No Iterations 1 time step continuity errors : sum local = 5.10374772441e09, global = 2.03780605143e10, cumulative = 5.16471384195e10 DILUPBiCG: Solving for omega, Initial residual = 0.000574220023228, Final residual = 6.14969308295e08, No Iterations 1 bounding omega, min: 51.9537160506 max: 2783990.47407 average: 3215.46773736 DILUPBiCG: Solving for k, Initial residual = 0.00126294320421, Final residual = 1.596491519e07, No Iterations 1 smoothSolver: Solving for Ux, Initial residual = 4.0069355444e06, Final residual = 2.66756177809e08, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 2.81168384134e05, Final residual = 2.22995109099e07, No Iterations 1 GAMG: Solving for p, Initial residual = 5.20087345873e06, Final residual = 4.31188201265e06, No Iterations 1 GAMG: Solving for p, Initial residual = 4.43988865995e06, Final residual = 3.56541527305e06, No Iterations 1 GAMG: Solving for p, Initial residual = 3.65422303439e06, Final residual = 3.04594172644e06, No Iterations 1 time step continuity errors : sum local = 3.55003853738e09, global = 1.32507266627e10, cumulative = 6.48978650822e10 GAMG: Solving for p, Initial residual = 3.11340357727e06, Final residual = 2.66630235476e06, No Iterations 1 GAMG: Solving for p, Initial residual = 2.71542382035e06, Final residual = 2.3583809352e06, No Iterations 1 GAMG: Solving for p, Initial residual = 2.39680524373e06, Final residual = 9.82034988664e09, No Iterations 114 time step continuity errors : sum local = 1.14455927733e11, global = 1.35390645433e14, cumulative = 6.48992189887e10 DILUPBiCG: Solving for omega, Initial residual = 0.571814369085, Final residual = 5.85579618545e09, No Iterations 2 DILUPBiCG: Solving for k, Initial residual = 0.0615167993133, Final residual = 1.51064699096e07, No Iterations 1 ExecutionTime = 45.47 s ClockTime = 45 s HTML Code:
Courant Number mean: 0 max: 0.00165125113506 velocity magnitude: 7.125 deltaT = 1.19904076739e06 Time = 1.19904076739e06 DILUPBiCG: Solving for Ux, Initial residual = 1, Final residual = 5.2997904141e08, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 1, Final residual = 3.92819713926e07, No Iterations 1 GAMG: Solving for p, Initial residual = 1, Final residual = 9.8940958311e06, No Iterations 286 GAMG: Solving for p, Initial residual = 0.00492661366781, Final residual = 9.69441981314e06, No Iterations 63 GAMG: Solving for p, Initial residual = 0.000438655867845, Final residual = 9.56808439715e06, No Iterations 20 time step continuity errors : sum local = 1.11209229401e08, global = 6.13442805401e11, cumulative = 6.13442805401e11 GAMG: Solving for p, Initial residual = 0.000896181004493, Final residual = 9.94003026082e06, No Iterations 19 GAMG: Solving for p, Initial residual = 0.000114038891381, Final residual = 9.28235138726e06, No Iterations 6 GAMG: Solving for p, Initial residual = 3.1145114298e05, Final residual = 8.00575569478e06, No Iterations 3 time step continuity errors : sum local = 9.31716491533e09, global = 3.3954703335e10, cumulative = 4.0089131389e10 DILUPBiCG: Solving for omega, Initial residual = 0.00276574193747, Final residual = 1.18013315195e06, No Iterations 2 DILUPBiCG: Solving for k, Initial residual = 0.999999998834, Final residual = 2.04821821548e06, No Iterations 3 DILUPBiCG: Solving for Ux, Initial residual = 0.00328256439747, Final residual = 9.22262556873e08, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 0.0166023384362, Final residual = 3.69188648857e07, No Iterations 1 GAMG: Solving for p, Initial residual = 0.000108687134005, Final residual = 9.10996557958e06, No Iterations 15 GAMG: Solving for p, Initial residual = 7.68663677355e05, Final residual = 8.51764028408e06, No Iterations 5 GAMG: Solving for p, Initial residual = 2.31392311457e05, Final residual = 7.02587951765e06, No Iterations 3 time step continuity errors : sum local = 8.19377779615e09, global = 3.64619989004e10, cumulative = 3.62713248856e11 GAMG: Solving for p, Initial residual = 1.20610790822e05, Final residual = 7.23892901734e06, No Iterations 1 GAMG: Solving for p, Initial residual = 7.46956457073e06, Final residual = 5.56488553551e06, No Iterations 1 GAMG: Solving for p, Initial residual = 5.92499120684e06, Final residual = 4.42706445922e06, No Iterations 1 time step continuity errors : sum local = 5.16293280986e09, global = 2.15140122523e10, cumulative = 2.51411447409e10 DILUPBiCG: Solving for omega, Initial residual = 0.000574220017969, Final residual = 6.47609273803e08, No Iterations 1 bounding omega, min: 52.4826853551 max: 2783990.47407 average: 3215.47457647 DILUPBiCG: Solving for k, Initial residual = 0.00126292944634, Final residual = 1.71053486661e07, No Iterations 1 smoothSolver: Solving for Ux, Initial residual = 4.01826769091e06, Final residual = 2.67134825097e08, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 2.80798370421e05, Final residual = 2.23039106906e07, No Iterations 1 GAMG: Solving for p, Initial residual = 5.25261897253e06, Final residual = 4.27996390355e06, No Iterations 1 GAMG: Solving for p, Initial residual = 4.39468304143e06, Final residual = 3.54945868384e06, No Iterations 1 GAMG: Solving for p, Initial residual = 3.63252709134e06, Final residual = 3.00424092978e06, No Iterations 1 time step continuity errors : sum local = 3.50234785006e09, global = 1.19404746341e10, cumulative = 1.32006701068e10 GAMG: Solving for p, Initial residual = 3.06460438258e06, Final residual = 2.61585694502e06, No Iterations 1 GAMG: Solving for p, Initial residual = 2.6597484296e06, Final residual = 2.31236874164e06, No Iterations 1 GAMG: Solving for p, Initial residual = 2.34525336265e06, Final residual = 9.72102473968e09, No Iterations 108 time step continuity errors : sum local = 1.13327805178e11, global = 6.13248770843e15, cumulative = 1.3200056858e10 DILUPBiCG: Solving for omega, Initial residual = 0.582577485522, Final residual = 5.74478651409e09, No Iterations 2 DILUPBiCG: Solving for k, Initial residual = 0.064033662544, Final residual = 1.52036103476e07, No Iterations 1 ExecutionTime = 121.82 s ClockTime = 123 s 

August 31, 2013, 13:14 

#5  
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Bruno Santos
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Hi mechy,
On this thread of yours, you have more information beyond the information you've sent me over private messages. So let's continue the discussion here. OK, there are no reasons that I can see that this would be a hardware related limitation, since you are using a real machine with a dual core processor. Now, looking at your previous post, I saw this line and another similar to it, on both serial and parallel runs: Quote:
Therefore, I think the problem you are seeing is part of a limitation on either OpenFOAM or on the MPI to be able to handle invalid numbers, such as NaN and Inf. So, first you should fix whichever problem your case has in the setup of the boundaries or the mesh. Best regards, Bruno
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August 31, 2013, 15:07 

#6 
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Dear Bruno
the results for above threads are for different test case. I test a flow around a cylinder (for laminar condition ) and in serial run first time step consume 6 sec and in parallel run it consume 15 sec I have attached the test case the results of solver are shown as follow decomposePar HTML Code:
Calculating distribution of cells Selecting decompositionMethod simple Finished decomposition in 0.12 s Calculating original mesh data Distributing cells to processors Distributing faces to processors Distributing points to processors Constructing processor meshes Processor 0 Number of cells = 86400 Number of faces shared with processor 1 = 539 Number of processor patches = 1 Number of processor faces = 539 Number of boundary faces = 173805 Processor 1 Number of cells = 86400 Number of faces shared with processor 0 = 539 Number of processor patches = 1 Number of processor faces = 539 Number of boundary faces = 173655 Number of processor faces = 539 Max number of cells = 86400 (0% above average 86400) Max number of processor patches = 1 (0% above average 1) Max number of faces between processors = 539 (0% above average 539) Time = 0 Processor 0: field transfer Processor 1: field transfer End. HTML Code:
PIMPLE: no residual control data found. Calculations will employ 3 corrector loops Starting time loop Courant Number mean: 3.39655234767e07 max: 0.000161747173295 deltaT = 1.19904076739e06 Time = 1.19904076739e06 PIMPLE: iteration 1 DILUPBiCG: Solving for Ux, Initial residual = 1, Final residual = 6.96035197586e10, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 1, Final residual = 7.87886990224e15, No Iterations 1 GAMG: Solving for p, Initial residual = 1, Final residual = 9.71073975512e06, No Iterations 30 time step continuity errors : sum local = 7.90960050679e12, global = 9.2538434098e18, cumulative = 9.2538434098e18 PIMPLE: iteration 2 DILUPBiCG: Solving for Ux, Initial residual = 6.90125739808e05, Final residual = 7.59669009166e11, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 4.41526111253e06, Final residual = 4.41526111253e06, No Iterations 0 GAMG: Solving for p, Initial residual = 0.00166409642772, Final residual = 9.35997918544e06, No Iterations 9 time step continuity errors : sum local = 7.90865831711e09, global = 1.85991682847e14, cumulative = 1.86084221281e14 PIMPLE: iteration 3 smoothSolver: Solving for Ux, Initial residual = 1.59677155046e05, Final residual = 8.70000414415e09, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.77933888357e05, Final residual = 7.67220185903e09, No Iterations 1 GAMG: Solving for p, Initial residual = 0.000101226876292, Final residual = 9.63072794241e09, No Iterations 22 time step continuity errors : sum local = 8.15010081926e12, global = 8.61571179427e17, cumulative = 1.85222650102e14 ExecutionTime = 14.05 s ClockTime = 15 s HTML Code:
Starting time loop Courant Number mean: 3.39655234767e07 max: 0.000161747173295 deltaT = 1.19904076739e06 Time = 1.19904076739e06 PIMPLE: iteration 1 DILUPBiCG: Solving for Ux, Initial residual = 1, Final residual = 2.30051737223e15, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 1, Final residual = 7.87886713367e15, No Iterations 1 GAMG: Solving for p, Initial residual = 1, Final residual = 5.69328754714e06, No Iterations 19 time step continuity errors : sum local = 4.63730214029e12, global = 4.80640009863e17, cumulative = 4.80640009863e17 PIMPLE: iteration 2 DILUPBiCG: Solving for Ux, Initial residual = 6.89082529956e05, Final residual = 5.13244948403e12, No Iterations 1 DILUPBiCG: Solving for Uy, Initial residual = 4.39821658069e06, Final residual = 4.39821658069e06, No Iterations 0 GAMG: Solving for p, Initial residual = 0.00166385360517, Final residual = 9.97281656696e06, No Iterations 9 time step continuity errors : sum local = 8.42646538031e09, global = 1.65728690322e12, cumulative = 1.65733496722e12 PIMPLE: iteration 3 smoothSolver: Solving for Ux, Initial residual = 1.59227489748e05, Final residual = 8.68277817384e09, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.77504455575e05, Final residual = 7.64446712243e09, No Iterations 1 GAMG: Solving for p, Initial residual = 0.000100775380721, Final residual = 5.89445176647e09, No Iterations 15 time step continuity errors : sum local = 4.98821054588e12, global = 5.34660543394e18, cumulative = 1.65734031382e12 ExecutionTime = 6.26 s ClockTime = 6 s 

August 31, 2013, 15:30 

#7 
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Bruno Santos
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Hi mechy,
Ah, good thing you provided the case! The problem is that you had wrongly configured the GAMG for the pressure. By using this: Code:
nCellsInCoarsestLevel 20; I used as reference this post: http://www.cfdonline.com/Forums/ope...tml#post295166 post #10 Best regards, Bruno
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August 31, 2013, 16:14 

#8 
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Dear Bruno
thanks so much for your answer also, it is work well for me and at now the parallel is faster than the serial in some threads I read that the Code:
nCellsInCoarsestLevel and at now I think it is not correct do you know what is the best value for nCellsInCoarsestLevel does it have fixed value for all number of mesh cells ? also, in my turbulent runs the value of omega has the order of 2e6 is it true ? Best Regards 

August 31, 2013, 17:18 

#9  
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Bruno Santos
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Hi mechy,
Quote:
Quote:
I usually suggest that you scale down and/or simplify your case first, to ensure if things are working properly. Best regards, Bruno
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September 1, 2013, 01:58 

#10 
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Dear Bruno
thanks so much for your reply what is the maximum value of nCellsInCoarsestLevel which is used in your problems ? and can you explain that how should I do trialanderror to select best value for it ? Best Regards 

September 1, 2013, 06:57 

#11  
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Hi mechy,
I rarely use GAMG. What I mean by trialanderror is what Martin said several years ago and I quote: Quote:
Bruno
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September 1, 2013, 10:27 

#12 
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Hi Bruno
thanks very much is there any command in decomposePar that forces whole of a boundary fall in a processor ? in other words if I have a boundary with name plate and this plate is in the middle of mesh , for running with 2 processor ,how can I set all of plate in a one processor Best Regards 

September 1, 2013, 10:36 

#13 
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Look at the file "applications/utilities/parallelProcessing/decomposePar/decomposeParDict": https://github.com/OpenFOAM/OpenFOAM...composeParDict
It's the "preservePatches" option, but I think it only works for cyclic patches.
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September 1, 2013, 11:10 

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thanks for your answer
I test it but it does not work for other patches Best Regards 

July 22, 2014, 16:51 

#15 
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miladrakhsha
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Hi Bruno
I believe that I have the same problem yet in spite of all my efforts and after a couple of days that I have been searching for a solution I was not able to figure this out. I am experiencing with a 3D mesh which has 3 million cells and I am using simpleFoam solver. For a serial simulation of a 2D case (100,000 cells) of almost the same problem I spent very short time to get the results.(about 23 hours for 20003000 iteration) Nonetheless, for the 3D case the simulation proceeds disappointingly slow. It has been running for 4 days and it just got to 1000th iteration. I have attached my fvSolution, fvSchemes and decomposeParDict file to this post for more information. Also a small part of the log file of simpleFoam is as follow: (About one of your comment I read in this page regarding negative omega I should mention that unfortunately this is not the problem in my case. I have experienced the same thing in the 2D case though it does not cause a problem in that 2D case ) Time = 1060 DILUPBiCG: Solving for Ux, Initial residual = 0.000158359, Final residual = 5.31028e07, No Iterations 2 DILUPBiCG: Solving for Uy, Initial residual = 0.000194343, Final residual = 6.61693e07, No Iterations 2 DILUPBiCG: Solving for Uz, Initial residual = 0.00132841, Final residual = 8.17812e07, No Iterations 3 GAMG: Solving for p, Initial residual = 0.373173, Final residual = 0.0827443, No Iterations 1000 time step continuity errors : sum local = 0.00194834, global = 7.21998e15, cumulative = 2.62773e13 DILUPBiCG: Solving for omega, Initial residual = 0.00529106, Final residual = 1.44872e05, No Iterations 1 bounding omega, min: 905.065 max: 52459.6 average: 61.2743 DILUPBiCG: Solving for k, Initial residual = 0.000462427, Final residual = 9.88307e07, No Iterations 3 bounding k, min: 3.68267 max: 129.378 average: 0.447474 ExecutionTime = 337633 s ClockTime = 338023 s forceCoeffs forceCoeffs output: Cm = 0.232539 Cd = 0.156459 Cl = 0.0756161 Cl(f) = 0.194731 Cl(r) = 0.270347 Time = 1061 DILUPBiCG: Solving for Ux, Initial residual = 0.000168488, Final residual = 1.1647e06, No Iterations 2 DILUPBiCG: Solving for Uy, Initial residual = 0.000200027, Final residual = 5.57576e07, No Iterations 2 DILUPBiCG: Solving for Uz, Initial residual = 0.00151299, Final residual = 9.32168e06, No Iterations 2 GAMG: Solving for p, Initial residual = 0.383116, Final residual = 0.0607197, No Iterations 1000 time step continuity errors : sum local = 0.00133519, global = 1.74997e15, cumulative = 2.64523e13 DILUPBiCG: Solving for omega, Initial residual = 0.0133203, Final residual = 1.47131e05, No Iterations 1 bounding omega, min: 756.761 max: 59953.8 average: 61.2054 DILUPBiCG: Solving for k, Initial residual = 0.000462061, Final residual = 2.65904e06, No Iterations 2 bounding k, min: 1.97769 max: 129.043 average: 0.447068 ExecutionTime = 338002 s ClockTime = 338392 s forceCoeffs forceCoeffs output: Cm = 0.244678 Cd = 0.160826 Cl = 0.0467811 Cl(f) = 0.221287 Cl(r) = 0.268068 I would really appreciate your helpor anybody who can help  in advance 

July 22, 2014, 16:57 

#16 
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miladrakhsha
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Also I always had this question for which I could not find an answer. It might seem a stupid question but I have some experiences with fluent and as I remember my speed up in fluent was very close to the number of processors that I used. However, in openFoam it seems that for some reasons this is not the case and speed up coefficient is far less than number of processors.
I would be grateful if you could answer this question or suggest a useful link for this purpose. 

July 28, 2014, 03:08 

#17 
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Hi
the number of processors and also nCellsInCoarsestLevel 500; is very high set the following values: nCellsInCoarsestLevel > 20 to 50 numberOfSubdomains > number of your machine processor 

July 28, 2014, 17:40 

#18  
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miladrakhsha
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Quote:
I have used nCellsInCoarsestLevel=sqrt(ncells) as you and Bruno mentioned in this topic. Also when I decrease this parameter there is not any obvious change in pressure solver speed. In addition, as I am working with a machine with 32 CPUs so the numberOfSubdomains that you mentioned is less than number of CPUs in my simulation. Based on my experience, in my parallel simulation, PCG (preconditioned Conjugate Gradient method) do a better job that GAMG solver although GAMG solver is faster in serial simulations. Thank you Milad 

August 17, 2016, 18:19 
Update

#19 
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Victor
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I just had the same problem in OpenFoam 4 but it was a silly mistake. I was typing
mpirun np 10 pimpleFoam the correct is mpirun np 10 pimpleFoam parallel I got confused because, if you use the first command line, your case still runs but much slower. So you can see the results and everything. I hope that some of you can save some time with this post Cheers Victor 

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