How exactly does AsParallel work?
Solution 1
Task Parallel Library cares about the static type of the sequence. It should be IParallelEnumerable<T>
for the operations to be handled by the TPL. You are casting the collection back to IEnumerable<T>
when you call Test
. Therefore, the compiler will resolve .Where
call on the sequence to System.Linq.Enumerable.Where
extension method instead of the parallel version provided by the TPL.
Solution 2
(Updating for .NET4 since this question ranks pretty high in a Google search for AsParallel())
Just a few changes will allow your example to work as I imagine you expected.
Change List<int> list = 0.UpTo(4);
to var list = Enumerable.Range(0, 4);
Your example would work if you added a function overload with a signature that takes a ParallelQuery...
private static void Test(ParallelQuery<int> input)
{
var timer = new Stopwatch();
timer.Start();
int size = input.Count();
if (input.Where(IsOdd).Count() != size / 2)
{
throw new Exception("Failed to count the odds");
}
timer.Stop();
Console.WriteLine("Tested " + size + " numbers in " + timer.Elapsed.TotalSeconds + " seconds");
}
Alternatively, you could use LINQ syntax....
private static void Test(ParallelQuery<int> list)
{
var timer = Stopwatch.StartNew();
if ((from n in list.AsParallel()
where IsOdd(n)
select n).Count() != (list.Count() / 2))
{
throw new Exception("Failed to count the odds");
}
Console.WriteLine("Tested " + list.Count() + " numbers in " + timer.Elapsed.TotalSeconds + " seconds");
}
Hope this helps someone!
Solution 3
As Parallel works by putting your stuff into the ThreadPool. Also, how many cores do you have? If you're working on a single core machine that will still take about 4s to run.
Comments
-
ripper234 almost 2 years
It doesn't seem to do squat for the following test program. Is this because I'm testing with a small list?
static void Main(string[] args) { List<int> list = 0.UpTo(4); Test(list.AsParallel()); Test(list); } private static void Test(IEnumerable<int> input) { var timer = new Stopwatch(); timer.Start(); var size = input.Count(); if (input.Where(IsOdd).Count() != size / 2) throw new Exception("Failed to count the odds"); timer.Stop(); Console.WriteLine("Tested " + size + " numbers in " + timer.Elapsed.TotalSeconds + " seconds"); } private static bool IsOdd(int n) { Thread.Sleep(1000); return n%2 == 1; }
Both versions take 4 seconds to run.