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Hyperthreading & Number of Cores. Parallel computing toolbox

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Matlab2010
Matlab2010 on 9 Apr 2014
Commented: Shifan Gao on 8 May 2018
I am using:
MATLAB Version: 8.3.0.532 (R2014a)
MATLAB License Number:
Operating System: Microsoft Windows 8.1 Version 6.3 (Build 9600)
Java Version: Java 1.7.0_11-b21 with Oracle Corporation Java HotSpot(TM) 64-Bit Server VM mixed mode
----------------------------------------------------------------------------------------------------
MATLAB Version 8.3 (R2014a)
Parallel Computing Toolbox Version 6.4 (R2014a)
I check my number of cores;
feature('numcores')
MATLAB detected: 4 physical cores.
MATLAB detected: 8 logical cores.
MATLAB was assigned: 8 logical cores by the OS.
MATLAB is using: 4 logical cores.
MATLAB is not using all logical cores because hyper-threading is enabled.
ans =
4
I also type;
>> p = parpool(8)
p.NumWorkers
Error using parpool (line 99)
You requested a minimum of 8 workers, but the cluster "local" has the NumWorkers property set to allow a maximum of 4 workers. To run a communicating job on
more workers than this (up to a maximum of 512 for the Local cluster), increase the value of the NumWorkers property for the cluster. The default value of
NumWorkers for a Local cluster is the number of cores on the local machine.
How can I allocate all 8 logical cores for matlab to use when running parfor.m for example? i.e. how can I make MATLAB use/ not use hyperthreading?
thanks

Accepted Answer

Jill Reese
Jill Reese on 9 Apr 2014
Under the Parallel Menu, choose "Manage Cluster Profiles...". This will pull up the Cluster Profile Manager window. Choose the "local" Cluster Profile, click the "Edit" button, and change the value for the NumWorkers property to 8 (or more). That field should be the second one from the top of the window, I think.
  9 Comments
Kay Gemba
Kay Gemba on 19 May 2016
Jeff, the 'additional' 4 logical cores share their resources with the actual 4 cores. Hence using 8 will not help when maxing out the resources. Manufactures thus can then sell you '8' cores ;)
~kai

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More Answers (1)

John Videtich
John Videtich on 13 Apr 2018
To resurrect an old thread, I ran a test on a machine with 24 cores / 48 logical (Xeon Gold 6146 x 2).
With 24 workers my calculation was taking 25.5 seconds. With 36 workers it was down to 21 seconds. With 40 workers it was down to 20.3. With 46 workers it was down to 19.8 seconds.
So it would seem hyperthreading does provide a real benefit - in this case, 22% reduction. I'm sure it's workload-dependent.
  1 Comment
Shifan Gao
Shifan Gao on 8 May 2018
Add on: my task takes 33 h with 4 pools and 19 h with 8 pools (i7 4 core). So new edition gives support to that kind of setting?

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