How to calculate completion percentage in the parallel for?

Question

With for calculating the completion percentage is easy:

for (int i = 0; i < 10; i++)
{
    for (int j = 0; j < 10; j++)
    {
        // Do somethings...
        Console.WriteLine("{0}% Done!", i * 10 + j + 1);
    }
}

Output:

1% Done!
2% Done!
3% Done!
4% Done!
5% Done!
6% Done!
7% Done!
8% Done!
9% Done!
10% Done!
11% Done!
12% Done!
13% Done!
...
93% Done!
94% Done!
95% Done!
96% Done!
97% Done!
98% Done!
99% Done!
100% Done!

---

But I don't have any idea about the calculating completion percentage with Parallel.For:

Parallel.For(0, 10, i =>
{
    Parallel.For(0, 10, j =>
    {
        // Do somethings...
        Console.WriteLine("{0}% Done!", i * 10 + j + 1);
    });
});

Output:

1% Done!
2% Done!
3% Done!
51% Done!
52% Done!
53% Done!
54% Done!
55% Done!
56% Done!
57% Done!
58% Done!
59% Done!
60% Done!
4% Done!
5% Done!
6% Done!
...
69% Done!
70% Done!
71% Done!
72% Done!
73% Done!
74% Done!
75% Done!
76% Done!
77% Done!
78% Done!
79% Done!
80% Done!
36% Done!
37% Done!
38% Done!
39% Done!
40% Done!
35% Done!

The output is not shown the correct completion percentage.

How to calculate completion percentage in the Parallel.For without reduce the performance?

Answer 1

You can use Interlocked.Increment method for that:

int progress = 0;
Parallel.For(0, 10, i =>
{
    Parallel.For(0, 10, j =>
    {
        // Do somethings...
        Console.WriteLine("{0}% Done!", Interlocked.Increment(ref progress));
    });
});

There will be one increment per iteration of the inner loop. Some printouts may appear out of sequence - specifically, when a thread is preempted after obtaining the new value of the progress but before the printing is complete.

Answer 2

For a simple case like this when you know that you're processing exactly 100 items, just have an outer variable that you (thread safely) increment and report on:

int progress = 0;
Parallel.For(0, 10, i =>
{
    Parallel.For(0, 10, j =>
    {
        // Do somethings...
        Console.WriteLine("{0}% Done!", Interlocked.Increment(ref progress));
    });
});

But note that you may still get some values reported out of order (since parallel tasks will perform the increment and then have to compete for access to the Console, which is an exclusive resource)