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Starting Julia with Julia -p 2 provides 1 worker
11 posts by 3 authors  



Iacopo Poli 	

Apr 19


Hi,

I'm trying to start Julia with more than one worker, but if I type in the terminal for example "julia -p 2", then in the REPL nworkers() returns 1.
I have version 0.5.0-dev+3488 and a Intel Core i5 (Macbook Pro Mid 2012).  

Running system_profiler:

"system_profiler SPHardwareDataType
Hardware:

 Hardware Overview:



      Model Name: MacBook Pro

      Model Identifier: MacBookPro9,2

      ...

      Total Number of Cores: 2

      ...

"

So I should be able to start Julia with two workers... Any guess why this isn't working?
 
This message has been deleted.



Greg Plowman 	

Apr 19




julia -p 2 will start Julia with 2 processes.
nprocs() will return 2
nworkers() will return 1 (1 less than nprocs())

http://docs.julialang.org/en/release-0.4/stdlib/parallel/?highlight=nworkers#Base.nworkers

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Andre Bieler 	

Apr 19


As Greg said, the total number of workers is one less than the number of available processes.
There is always one master process (with id = 1, and not considered a worker) and the remaining workers.

So for julia -p 3 you will get one master process and two workers.

The documentation may be misleading in this context, as it says:
Starting with julia -p n provides n worker processes on the local machine. 

But nworkers() is actually n-1.
However, if you do a @everywhere, the expression will be executed on all n
processes.
 


Iacopo Poli 	

Apr 19


All of this seems very reasonable... Except for the fact that also nprocs() is returning 1 after calling julia -p 2 :D

I call julia from terminal using export PATH in bash_profile, but the behavior is the same if I go in the directory and call ./Julia -p 2

Plus, I think I saw n workers after a call at julia -p n in Julia 0.4 on another machine, but not sure about it...

(btw the doc really is misleading, maybe should be restated in more clearly, but it's a minor issue)
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Andre Bieler 	

Apr 19


Huh.. I just tried julia -p 2 on version 0.4.5 and it actually starts with 3 processes and 2 workers.
So the docs are correct.

Sorry i cant help with the 0.5 version..

 


Iacopo Poli 	

Apr 20


I downloaded version 0.4.5, again calling it with -p 2 returns 1 process and 1 worker... I really can't understand why
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Greg Plowman 	

Apr 20


Sorry, I can't really help you with command line julia -p 2
But what happens when you call addprocs() from REPL?
Also, what is the value of CPU_CORES (typed at REPL)?

 
This message has been deleted.



Iacopo Poli 	

Apr 21


Calling addprocs() from REPL with for example CPU_CORES as argument returns 5 procs and 4 workers as expected, CPU_CORES returns 4 (physical cores are 2, but logical ones are 4). Also rmprocs(n) works fine, it lets me remove all procs but the first as I would expect.
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Iacopo Poli 	

Apr 28


A friend on mine encounters the same problem with another Macbook Pro model and El Captain OS. "julia -p N" doesn't work from terminal but addprocs() works as a charm