Optimize performance of a Parallel.For

Question

I have replaced a for loop in my code with a Parallel.For. The performance improvement is awesome (1/3 running time). I've tried to account for shared resources using an array to gather result codes. I then process the array out side the Parallel.For. Is this the most efficient way or will blocking still occur even if no iteration can ever share the same loop-index? Would a CompareExchange perform much better?

int[] pageResults = new int[arrCounter];
Parallel.For(0, arrCounter, i =>
{
   AlertToQueueInput input = new AlertToQueueInput();
   input.Message = Messages[i];

   pageResults[i] = scCommunication.AlertToQueue(input).ReturnCode;
});

foreach (int r in pageResults)
{
    if (r != 0 && outputPC.ReturnCode == 0) outputPC.ReturnCode = r;
}

Answer 1

It depends on whether you have any (valuable) side-effects in the main loop.

When the outputPC.ReturnCode is the only result, you can use PLINQ:

outputPC.ReturnCode = Messages
    .AsParallel()
    .Select(msg =>    
    {
       AlertToQueueInput input = new AlertToQueueInput();
       input.Message = msg;        
       return scCommunication.AlertToQueue(input).ReturnCode;
    })
    .FirstOrDefault(r => r != 0);

This assumes scCommunication.AlertToQueue() is thread-safe and you don't want to call it for the remaining items after the first error.

Note that FirstOrDefault() in PLinq is only efficient in Framework 4.5 and later.

Answer 2

You could replace:

foreach (int r in pageResults)
{
    if (r != 0 && outputPC.ReturnCode == 0) outputPC.ReturnCode = r;
}

with:

foreach (int r in pageResults)
{
    if (r != 0)
    {
        outputPC.ReturnCode = r;
        break;
    }
}

This will then stop the loop on the first fail.