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code, there are some problematic cases you would have to worry about The native thread pool, for example, will not interoperate with the CLR's garbage collector (GC); the GC needs to block threads during a collection, which the thread pool will respond to by introducing additional threads to run work This can lead to some interesting problems There are bound to be other issues that you'd encounter by going down this path, so I would strongly advise against it I will also mention that a lot of people favor writing custom thread pools (You will find one later in this chapter ) The reasons are numerous The platform thread pools are black boxes to most people, and, when it comes to scheduling work, black boxes can be intimidating You'd like to know precisely how and when work will run, and what decisions went into determining those things This chapter should help to eliminate the mys tery Once you understand how the decisions are made, however, you might legitimately disagree with the policies There are some features to control these decisions, but not enough to satisfy every requirement One last reason people roll their own is that the thread pool idea, at face value, is fairly simple to understand, and writing one is a good way to get initiated to basic threading and synchronization concepts I recommend that you recognize this as what it is: a learning exercise and not an attempt to build product quality code that you will ship If you decide, after much analysis, that you must write your own thread pool, just know that it can be extremely costly It typically starts off look ing very simple and, over time, grows in complexity as various corner cases are discovered Reading this chapter should convince you of this And you may introduce some odd interactions between yours and the other thread pools in the system along the way Since many platform components implicitly use the existing pools, you're apt to end up in a resource battle with those other platform components In 1 2, Parallel Containers, we will examine some more advanced queuing mechanisms for thread pool style work management Namely, we'll take a look at a highly efficient work stealing queue that does even better than the platform's thread pools for most cases While this is an inter esting topic from an I-have-to-know-everything-there-is-to-know-about concurrency standpoint, the platform thread pools are suitable for almost
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everybody who needs to write real programs So don't turn up your nose just yet without even reading the pages that follow If you do end up creating your own thread pool, however, that section is a must read
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Common Features Each of the three thread pools-the Windows Vista, legacy Win32, and CLR thread pool-offer very similar functionality There are a handful of features that any one pool offers over another, and some dramatic differences in the thread management policies and APls used to access the features, but we'll cover how you access four basic features with each of the particular pools These features are: work callbacks, I / O callbacks, timer callbacks, and wait registration callbacks Let's review each at a high level before moving on
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The simplest functionality offered is the ability to queue a work callback to execute asynchronously on a thread pool thread A single work callback maps directly to the notion of a concurrent task In the case of native code, this callback is represented by a function pointer, and in managed code, a delegate; both also accept an optional state argument The callback code pointer plus the state argument form a closure Each of the thread pool implementations maintains its own queue of work and a set of threads ded icated to executing work Queuing a work item places the callback into a queue that these threads monitor Eventually one of them will see it, dequeue the callback, invoke it, and then go back for more This is the least specialized and most frequently used feature of the pools
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