Selecting cores independently in a dual quad core machine

jalarron · September 2, 2009, 22:45

Hi all,

I need to select specifically the cores in a dual quad core Xeon machine, running fluent in SUSE SLED 10 Sp2.

I have 8 cores, and wanted to run 2 cases each using 4 cores. I've started one case with: fluent 3d -t4 & ;but as soon as I start the other case: fluent 3d -t4 &. The first case slows down immediately. So I'm assuming that the second command is picking the same or some of the cores of the first case.

Is there a way to select independently each core in fluent? I used to do this with the option cnf=n1,n2,n3,n4. But in that time I had each node as separated pc.

Cheers,
Javier

mystic_cfd · September 7, 2009, 17:06

Hi Javier,

Are you running 6.3.26? There was a problem that it would launch the second run on the same cores with that version of Fluent. If you use 6.3.35 or 12.0.16 it should not happen.

To set the cores manually:
- Start the first job to run (it will grab four cores for itself).
- start second Fluent session and load case and data
- set the process affinity with the TUI command:
(set-affinity "0 4 1 5 2 6 3 7")
This example will set a 4-partition parallel run to run on 4 cores; the data is arranged in pairs such that partition 0 is sent to core 4, partition 1 is sent to core 5 etc.

To see which cores are currently loaded, use top, and when top is running press "1". This expands the CPU section at the top to show individual cores. From the load status you will be able to see which cores are not occupied.

Hopes this helps.

cheers.

PS. if you are using older series Xeon (e.g. 52xx, 53xx, 54xx, 73xx) the bad news is that there will be very little speedup going from 4 to 8 cores due to memory bandwidth limitations. for newer series (55xx) this will not be the case. if licenses are a problem, it is best to run 2 threads per CPU (only use 2 of the 4 cores). It should be as fast as using all 4, and it uses half the licenses.

September 2, 2009, 22:45	Selecting cores independently in a dual quad core machine	#1
jalarron New Member Javier Larrondo Join Date: Jun 2009 Posts: 17 Blog Entries: 1 Rep Power: 16	Hi all, I need to select specifically the cores in a dual quad core Xeon machine, running fluent in SUSE SLED 10 Sp2. I have 8 cores, and wanted to run 2 cases each using 4 cores. I've started one case with: fluent 3d -t4 & ;but as soon as I start the other case: fluent 3d -t4 &. The first case slows down immediately. So I'm assuming that the second command is picking the same or some of the cores of the first case. Is there a way to select independently each core in fluent? I used to do this with the option cnf=n1,n2,n3,n4. But in that time I had each node as separated pc. Cheers, Javier

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Superlinear speedup in OpenFOAM 13	msrinath80	OpenFOAM Running, Solving & CFD	18	March 3, 2015 05:36
Dual Xeon PIV 3.8Ghz vs 2x Dual Core E5130 2.0 GHz	Michiel	Hardware	4	July 31, 2009 06:06
Parallel computing quad core	Prad	Main CFD Forum	13	February 9, 2009 14:28
Distributed Parallel on dual core remote machine	Justin	CFX	1	February 3, 2008 17:23
Questions about CPU's: quad core, dual core, etc.	Tim	FLUENT	0	February 26, 2007 14:02

September 7, 2009, 17:06		#2
mystic_cfd New Member HP Kritzinger Join Date: May 2009 Location: South Africa Posts: 25 Rep Power: 16	Hi Javier, Are you running 6.3.26? There was a problem that it would launch the second run on the same cores with that version of Fluent. If you use 6.3.35 or 12.0.16 it should not happen. To set the cores manually: - Start the first job to run (it will grab four cores for itself). - start second Fluent session and load case and data - set the process affinity with the TUI command: (set-affinity "0 4 1 5 2 6 3 7") This example will set a 4-partition parallel run to run on 4 cores; the data is arranged in pairs such that partition 0 is sent to core 4, partition 1 is sent to core 5 etc. To see which cores are currently loaded, use top, and when top is running press "1". This expands the CPU section at the top to show individual cores. From the load status you will be able to see which cores are not occupied. Hopes this helps. cheers. PS. if you are using older series Xeon (e.g. 52xx, 53xx, 54xx, 73xx) the bad news is that there will be very little speedup going from 4 to 8 cores due to memory bandwidth limitations. for newer series (55xx) this will not be the case. if licenses are a problem, it is best to run 2 threads per CPU (only use 2 of the 4 cores). It should be as fast as using all 4, and it uses half the licenses.