PLINQ和AsParallel（）

Question

What is the difference between this code:

int[] tab = new int[] { 1, 2, 3, 4, 5 };

List<int> result1 = (from t in tab
                     where t > 2
                     select t).AsParallel().ToList();

and this:

List<int> result2  = (from t in tab.AsParallel()
                      where t > 2
                      select t).ToList();

?

Answer 1

Everything after a call to AsParallel will be parallelized.

So in your first listing only the ToList operation will be parallelized as where in the second code sample, the entire query will be parallelized.

Answer 2

Let's go back .....

There are two strategies for partitioning work among threads: data parallelism and task parallelism.

Your are in : structured data parallelism zone. ( in plinq)

So the DATA which we deal with should be divided to threads.

Your first code is pointless.

Look at the syntax of AsParallel :

public static ParallelQuery AsParallel(
    this IEnumerable source
)

AsParallel should be applied on IEnumerable . ( like in both of your examples...but - )

The whole point here is to divide work to threads .

This divides DATA to threads which perform the SAME operation.

In the second code you DO divide DATA to threads which will perform same operation. ( hence you get better performance)

But in the first code , the whole "making cores hot operation" is done in single thread. ( where clause... ) - so you're missing the whole point

Notice - it would be worth something if :

...(from t in tab
    where t > 2
    select t).AsParallel().where(....)....