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)