3
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In a nutshell, how can I get this number:

enter image description here

I'm having trouble because $KernelID isn't a function and the FullForm is this crazy thing:

enter image description here

Motivation:

After you call CloseKernels[], the next time you LaunchKernels[] the new kernels don't start from one again:

enter image description here

In my parallel code I'd like to have access to the index of the kernel from 1 to $KernelCount, but I'm not sure how to efficiently get that:

offset = ToExpression@(StringCases[ToString[Kernels[][[1]]], NumberString][[1]])
(*Out[]= 17*)
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5
  • 1
    $\begingroup$ What if you kept a variable where you stored the output of LaunchKernels, and referenced the kernels by their index (1-4) in that variable? I am sure there are practical, low-level reasons why the kernels continue to increment. $\endgroup$
    – ktm
    Jan 23, 2020 at 20:29
  • 1
    $\begingroup$ Would 1 + Mod[$KernelID - 1, $KernelCount] do what you want? I.e., ParallelEvaluate[1 + Mod[$KernelID - 1, $KernelCount]] always gives, e.g., on a 4-core machine, {1,2,3,4}? $\endgroup$ Jan 24, 2020 at 4:18
  • $\begingroup$ That won't always give the same values {3, 1, 2}, {2, 3, 1}, ... $\endgroup$
    – user5601
    Jan 24, 2020 at 17:07
  • $\begingroup$ @user6014 the function I'm writing doesn't actually launch the kernels but uses ambient ones $\endgroup$
    – user5601
    Jan 24, 2020 at 17:27
  • $\begingroup$ I deleted my comment. I misunderstood what you said in the original version of the question. Sorry. $\endgroup$
    – Szabolcs
    Jan 24, 2020 at 20:41

1 Answer 1

3
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You can use the internal, undocumented function Parallel`Developer`KernelID to do this:

LaunchKernels[3];
Parallel`Developer`KernelID /@ Kernels[]

{1, 2, 3}

Close kernels and try again:

CloseKernels[];
LaunchKernels[4];
Parallel`Developer`KernelID /@ Kernels[]

{4, 5, 6, 7}

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