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Context

I am interested in doing parallel computation on a linux machine.

Odd behaviour

When I chose explicitly the number of kernels

(~)> math
Mathematica 12.0.0 Kernel for Linux x86 (64-bit)
Copyright 1988-2019 Wolfram Research, Inc.

In[1]:= LaunchKernels[8]

(* {KernelObject[1, local], KernelObject[2, local], …. , KernelObject[8, local]} *)

Whereas if I don't specify the number of Kernels

In[1]:= LaunchKernels[]

I get

wolfram: Command not found.

wolfram: Command not found.

...

wolfram: Command not found.

Question

Could anyone please reproduce what seems to be a (admittedly minor) Bug?

This seems to hold true for Mathematica 10.3 and 12.

Note that If one does not bother explicitly launching the kernels it does not work either

(~)> math
Mathematica 12.0.0 Kernel for Linux x86 (64-bit)
Copyright 1988-2019 Wolfram Research, Inc.

In[1]:= ParallelTable[Pause[1],{4}]//AbsoluteTiming
Launching kernels...

I get again

wolfram: Command not found.

wolfram: Command not found.

Finally note that on a mac it works as it should

In[1]:= ParallelTable[Pause[1], {4}] // AbsoluteTiming

Out[1]= {2.78291,{Null,Null,Null,Null}}

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  • $\begingroup$ Did you ever configure any kernels manually? Check $ConfiguredKernels (or $ConfiguredKernels // InputForm if needed). Kernel configuration can be done through the preferences dialog, so you may have set it even if you never used $ConfiguredKernel. $\endgroup$
    – Szabolcs
    Jan 12, 2020 at 9:46

2 Answers 2

Reset to default
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One difference between LaunchKernel[n] and LaunchKernels[] is that LaunchKernels[n] will launch n kernels on the local machine while LaunchKernels[] will launch whatever is in $ConfiguredKernels.

Most likely, you have configured subkernels manually through the graphical interface (Preferences dialog), and that configuration is no longer valid.

You can check/set the kernel configuration programmatically though $ConfiguredKernels or graphically in the preferences.

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  • $\begingroup$ Thanks for your reply. I don't use the frontend for this test. If I type In[1]:= $ConfiguredKernels Out[1]= {16 local kernels, 32 kernels on b6}. So this is not inconsistent with LaunchKernel[32] in principle, but in practice it seems to be. $\endgroup$
    – chris
    Jan 12, 2020 at 11:51
  • $\begingroup$ I should also say that this is not consistent with the number of cores available which is 40. In[9]:= RunProcess["lscpu", "StandardOutput"] Out[9]= Architecture: x86_64 CPU op-mode(s): 32-bit, 64-bit Byte Order: Little Endian CPU(s): 40 $\endgroup$
    – chris
    Jan 12, 2020 at 12:04
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    $\begingroup$ This confirms that you (or someone else) created a subkernel configuration manually. Notice the presence of non-local kernels. Revert this configuration to the default. reference.wolfram.com/language/ParallelTools/tutorial/… I am not sure how to permanently change the configuration without the front end, but assuming that it was changed manually, the same steps could be used to revert it ... $\endgroup$
    – Szabolcs
    Jan 12, 2020 at 15:25
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    $\begingroup$ One thought: Are you using the Lightweight Grid or some similar technology which automatically discovers available kernels on the network? I have never used it and have no experience with it. If you really did not ever change the subkernel configuration, maybe such a system did. Also: does the computer named b6 have the same version of Mathematica that you are using here, or a much older one that uses math instead of wolfram? $\endgroup$
    – Szabolcs
    Jan 12, 2020 at 16:04
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    $\begingroup$ @chris, rather than deleting, I would encourage you to self-answer with the likely problem, for future reference. $\endgroup$
    – MarcoB
    Jan 13, 2020 at 5:00
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As pointed by the accepted answer the problem arose because of artefacts from a (forgotten) past lightweight grid setting hidden within the .Mathematica directory .

The solution is to displace this directory and after a restart copy the mathpass back to License .

Incidentally it explains recurent misuses of the cluster which did not impress our system manager!

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