CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

Parallel running needs more iterations?

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree4Likes
  • 4 Post By arjun

Reply
 
LinkBack Thread Tools Display Modes
Old   March 26, 2011, 08:09
Default Parallel running needs more iterations?
  #1
New Member
 
Zoltan Hernadi
Join Date: Jul 2010
Posts: 12
Rep Power: 7
zhernadi is on a distinguished road
Dear OpenFOAM community,

I am testing parallel scalability on different machines and I found that sub-linear scaling may be caused by an increase of iterations.

In the attachments you can find my study based on 'cavity' case:
- iterations.png: Comparison of number of iterations: single/parallel
- iterations-case.zip: OpenFOAM 1.7.1 case (If you want to repeat my test, you will need gnuplot for plotting. Calculation should take less than 2 minutes.)

Questions:
1. Why does the solver need more iterations when solving a case in parallel?
2. Do you experience the same behavior in your CFD problems?
3. How can we change convergence criteria in order to have the same (or approx. same) number of iterations for the parallel run as we had for the single run?
4. Are there other possibilities to reduce this undesirable effect?

Thanks,
Zoltan
Attached Images
File Type: png iterations.png (11.6 KB, 57 views)
Attached Files
File Type: zip iterations-case.zip (5.3 KB, 8 views)
zhernadi is offline   Reply With Quote

Old   March 27, 2011, 04:19
Default
  #2
Senior Member
 
Alberto Passalacqua
Join Date: Mar 2009
Location: Ames, Iowa, United States
Posts: 1,894
Rep Power: 26
alberto will become famous soon enoughalberto will become famous soon enough
Does "pIters" refer to the iterations on the pressure equation?
__________________
Alberto Passalacqua

GeekoCFD - A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as live DVD/USB, hard drive image and virtual image.
OpenQBMM - An open-source implementation of quadrature-based moment methods
alberto is offline   Reply With Quote

Old   March 27, 2011, 07:01
Default
  #3
New Member
 
Máté Márton LOHÁSZ
Join Date: Jul 2010
Posts: 4
Rep Power: 7
lohasz is on a distinguished road
Yes "pIters" refers to the number of iterations on the pressure equation.

Máté (a colleague of Zoltán, this is the reason I know the answer as well)
lohasz is offline   Reply With Quote

Old   March 27, 2011, 21:38
Default
  #4
Senior Member
 
Arjun
Join Date: Mar 2009
Location: Nurenberg, Germany
Posts: 374
Rep Power: 10
arjun is on a distinguished road
with assumption that program has no bugs related to parallel programing.


Questions:
1. Why does the solver need more iterations when solving a case in parallel?

It depends on your matrix solver for pressure correction equation. Some matrix solvers require more iterations in parallel mode.


2. Do you experience the same behavior in your CFD problems?

It is known problem.

3. How can we change convergence criteria in order to have the same (or approx. same) number of iterations for the parallel run as we had for the single run?

Again it all depends on what matrix solver you are using. In some cases , it is not possible to change this behaviour. You have to increase iterations in matrix solver.


4. Are there other possibilities to reduce this undesirable effect?

yes, www.columbia.edu/~ma2325/poly.ps
arjun is offline   Reply With Quote

Reply

Tags
iterations, parallel

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Moving mesh Niklas Wikstrom (Wikstrom) OpenFOAM Running, Solving & CFD 122 June 15, 2014 06:20
Velocity blows up suddenly after 30,000+ iterations lordvon OpenFOAM 14 October 27, 2010 11:13
reactingFoam: adiabatic flame temperature rFkhemek OpenFOAM 2 July 31, 2009 06:42
IcoFoam parallel woes msrinath80 OpenFOAM Running, Solving & CFD 9 July 22, 2007 02:58
Automatic Mesh Motion solver michele OpenFOAM Running, Solving & CFD 10 September 26, 2005 08:21


All times are GMT -4. The time now is 03:49.