CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Why cores number in CFD simulation is preferred to be integer multiple of 4 for CFX?

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 11, 2015, 21:31
Question Why cores number in CFD simulation is preferred to be integer multiple of 4 for CFX?
  #1
Member
 
Pierre
Join Date: Mar 2015
Posts: 37
Rep Power: 11
Pierre1 is on a distinguished road
I noticed that people usually put 4, 8, 16or 32 cores to run a CFX simulation. The cores number is always an integermultiple of 4. This seems to be a convention. I seldom see people use 11 or 9cores to run a CFX simulation.

I’m wondering does this convention make anysense? Why?
Pierre1 is offline   Reply With Quote

Old   March 12, 2015, 04:50
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,665
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
You are right, it does not make much sense. A 5 computer cluster will simply run about 25% faster than a 4 computer cluster. So feel free to use odd numbers if you wish.

I think it is simply because benchmarks are often done using factors of 2, so you expect an approximate halving of computational time. There is not much point benchmarking parallel efficiency on a 31 way cluster compared to a 32 way cluster - you will get far more meaningful results comparing a 64 way cluster to a 32 way one.
ghorrocks is offline   Reply With Quote

Old   March 12, 2015, 16:46
Default
  #3
Senior Member
 
Edmund Singer P.E.
Join Date: Aug 2010
Location: Minneapolis, MN
Posts: 511
Rep Power: 20
singer1812 is on a distinguished road
Most cpus now days are quad cores. Probably the reason behind 4's. They just grab the whole chip when the run.

Our HPC is in clusters of 16, and it so happens that we have found that when we run with other software on the same node, it bogs everything down. Too lazy to trouble shoot, we usually just grab multiples of whole nodes when doing things.

Also, if you factor in hpc packs that ansys licenses, you will get another reason certain numbers of CPUs might be used.
singer1812 is offline   Reply With Quote

Old   March 23, 2015, 18:03
Default Core count
  #4
New Member
 
Daniel Wilde
Join Date: Jan 2014
Posts: 21
Rep Power: 12
QCFD is on a distinguished road
In my experience, it is a hardware dependent choice. if a cluster is comprised of 12 core chips, it is useful to grab a multiple of 12 so you do not use a partial chip. If you have a partial chip and someone else tries to use the other cores you can have issues. In some cases, the other cores of the partially used chip are not made available to other users so the just sit idle.
QCFD is offline   Reply With Quote

Old   March 23, 2015, 20:20
Default
  #5
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,665
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
For that to be the case it assumes that having multiple cores on a CPU working on the same job is better than having the multiple cores working on different jobs. Providing the jobs are equivalent (similar memory, similar parallelisation) in my experience it makes no difference what form the job takes it will load the system down about the same amount.

If you have a few cores idle then the active cores will run better as other bottlenecks (eg the memory and hard drive) are lighter loaded so the active cores work better. On top of that, many current Intel CPUs run at higher clock speeds when only one core is active which exaggerates this effect.

It is important to balance up the processes in a parallel run. If you have 4 identical 4 core CPU machines, and you send 4 processes to 3 machines but only 2 processes to 1 machine then it will not work too well as the loads are not balanced. It will run much better with either all machines having 4 processes, or even 3 processes.
ghorrocks is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Decomposing meshes Tobi OpenFOAM Pre-Processing 22 February 24, 2023 10:23
CFD Salary CFD Main CFD Forum 17 January 3, 2017 18:09
Help compiling code (newbie) thsb20 OpenFOAM Programming & Development 56 August 24, 2016 05:24
CFD Design...The CFD Future John C. Chien Main CFD Forum 20 November 20, 2015 00:40
Reactor simulation using CFD takes to the Internet Mark Brennan Main CFD Forum 0 April 16, 1999 06:57


All times are GMT -4. The time now is 11:39.