Parallelising Loops in .NET 4

Wed, January 7, 2009, 05:58 AM under ParallelComputing
Often the source of performance issues in our code is loops, e.g. while, for, foreach. With .NET 4 it becomes easy to make such code perform better by taking advantage of multiple cores.

For example, given:
IEnumerable<string> arr = ...
foreach (string item in arr){
// Do something
, we can parallelise it is as follows:
Parallel.ForEach<string>(arr, delegate (string item){
// Do something
, or the tidier directly equivalent version (dropping the superfluous generic which can be inferred and turning the anonymous method syntax into a lambda statement)
Parallel.ForEach (arr, (string item) =>{
// Do something

Visual Distinctions
Notice the obvious visual similarities that make it almost automatic to parallelise a loop: the only difference in the parallel version is the modification in the first line (rearranging the "arr" and "string item", which are the real pieces of information) and the fact that after the closing brace at the end there is a closing parenthesis and semicolon. The crucial visual observation here is that the body of the loop remains intact.

Why Does It Work
Let's drill into why the modified code compiles and why it is equivalent in intent (even if it is obvious to some). We turned a block of code into a library method call. The .NET 4 (mscorlib) library offers the static class Parallel that (among others) offer the ForEach method. One of its overloads (its simplest) accepts 2 parameters: a source IEnumerable of TSource and a body of code (in the form of the Action of TSource delegate, of course) that accepts a single parameter which is also of TSource, of course. The method will take the body and call it once for each element in the source. If you reread the last 2 sentences you'll find that is exactly what the original loop construct does as well. The real difference here is that the original runs serially (using only a single core) while the modified runs in parallel (using, by default, all cores).

How Does It Work
Those of you that don't like black magic boxes will ask: what does that method actually do inside in order to run things in parallel? My answer: what do you think it needs to do? Remember our friendly task-based programming model? The implementation of the methods of the static Parallel class uses Tasks (and specifically SelfReplicating tasks). Given that the implementation is subject to change and more importantly given that we have not shipped .NET 4 yet, I will not go into exactly how it uses the Tasks, but I leave that to your imagination (or to your decompiler-assisted exploration ;)).

Trivial Demo Example
In a .NET 4 Console project paste the following in the Program.cs file:
  static string[] arr = Directory.GetFiles(@"C:\Users\Public\Pictures\Sample Pictures", "*.jpg");
static void SimulateProcessing() {
static string TID {
get {
return " TID = " + Thread.CurrentThread. ManagedThreadId.ToString();
Now in the empty Main function paste the following:
    foreach (string ip in arr) {
Console.WriteLine(ip + TID);
Run it and notice how long it takes as well as that only one thread gets used of course and in Task Manager notice the CPU usage. Now change the loop construct so they are as follows:
Parallel.ForEach(arr, (string ip) => {
Console.WriteLine(ip + TID);
Re-run it and notice how much faster it runs and how the number of threads equals the number of cores on your machine and in Task Manager the CPU usage being at 100%.

Why Not Do It Automatically
Many that see this technology ask "Why not automatically change all loops to run parallelised?". The answer is that you cannot blindly apply a Parallel.ForEach wherever you have a foreach loop. If the body of the loop depends on some shared state, or if each loop iteration is not independent of every other iteration, then race conditions may arise by blindly parallelising. Ultimately, it is multiple threads that execute the body in parallel so there is no room for shared state etc. The static methods of the Parallel class have no magic to deal with that – it is still down to you. If you find yourself needing synchronization in the loop body, be sure to measure the performance because locks and such in a parallelisation scenario potentially negate (or severely limit) the benefits of parallelisation. It is for these reasons that parallelising a loop is an opt-in decision today that only you can make for your code.

A related question arises of why not embedding this functionality in the language (the obvious suggestion being introducing a pfor loop construct). The answer is that having it as a library offering instead of built-in to the language allows it to be used by *all* .NET languages instead of restricting it to a few. Also, once something is embedded into the language it typically stays there forever so we take great care about such decisions e.g. it is too early to tie C# or VB to the System.Threading.Tasks namespace.

For the distant imaginary future, we are thinking about automatically parallelising bits of code if we can (with hints from the application developer with regards to purity and side-effect-free regions of code) and also embedding parallel constructs deeper into the language. Too early to know if anything will come of that...

Can It Do More
Yes! We only saw above one of the overloads of one of the methods. Parallel.ForEach has ~20 other overloads, some of them taking up to 5 arguments and all of them having a return type too; what I am trying to say is that there is much more flexibility and richness even in this simple API. I encourage you to explore the other overloads and also the other two methods on the Parallel class: For and Invoke.