Break out of ParallelDo

Question

In a parallelised computation, I would like each kernel separately to break out of ParallelDo when it has finished going through the loop once. For a single kernel this works with Break[]. For example

Do[Print[i]; Break[], {i, 1, 100}]

Gives output

1

But if I try the same thing on ParallelDo (I have two kernels),

ParallelDo[Print[i]; Break[], {i, 1, 100}]

I get the strange output

1 (Kernel 2)

14 (Kernel 1)

27 (Kernel 2)

40 (Kernel 1)

53 (Kernel 2)

65 (Kernel 1)

77 (Kernel 2)

89 (Kernel 1)

while I would have expected just two outputs. It's essential that the two kernels break out separately, because my actual computation involves a random element such that running through the loop different times can take way more or less time. Thank you

Answer 1

Using

ParallelDo[Print[i]; Break[], {i, 1, 100}, Method -> "CoarsestGrained"]

or

ParallelDo[Print[i]; Break[], {i, 1, 100}, Method -> "EvaluationsPerKernel" -> 1]

should give you the desired behavior.
These options are explained and illustrated in the Options ▶ Method section of Parallelize.

Mathematica breaks the computation within ParallelDo into subunits. The default tries to balance evaluation size and number of evaluations. In your example these are 8 subunits, each starting individually not knowing that there occurred a Break[] for a smaller i before.

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For the example given in your comment below

Module[{counter = 0},
 ParallelTry[
  While[! 0.5 <= RandomReal[] <= 0.50000001, counter++]; 
  Print["Gonna break at trial " <> ToString[counter]], 
  Range[$ProcessorCount]]]

or

Module[{counter = 0},
 ParallelDo[
  While[! 0.5 <= RandomReal[] <= 0.50000001, counter++]; 
  Print["Gonna break at trial " <> 
ToString[counter]], {$ProcessorCount}]]

might be more suitable.