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Ordered PLINQ ForAll

Asked 2011-03-18T13:16:37.983
13

The msdn documentation about order preservation in PLINQ states the following about ForAll().

  • Result when the source sequence is ordered: Executes nondeterministically in parallel
  • Result when the source sequence is unordered: Executes nondeterministically in parallel

Does this mean that ordered execution of the ForAll method is never guaranteed?

I haven't used PLINQ before, but the following Code Review question seemed like an appropriate usage for it. At the bottom of my answer I write:

Events.AsParallel().AsOrdered().ForAll( eventItem =>
{
    ...
} );

After reading the documentation I believe the AsOrdered() wouldn't change anything?
I'm also suspecting the previous query can't replace a simple for loop where order is important?
Probably parallel calls to the StringBuilder will also occur, resulting in a wrong output?

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4

Does this mean that ordered execution of the ForAll method is never guaranteed?

Yes - order is not guaranteed.

The parallelisation means that the work is allocated to different threads and their separate outputs are then later combined.

If you need to order the output then don't use PLinq - or add some later step to put the ordering back in.

Also, if you are accessing objects like a StringBuilder from within the plinq execution, then please ensure that those objects are threadsafe - and also be aware that this thread safety may in fact make the plinq slower than the non-parallel linq.

answered 2011-03-18T13:22:18.477

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