Threading Advantages
- Prioritization - Individual threads priorities can be adjusted. This allows individual server tasks or individual connections to be given more or less CPU time.
- Encapsulation - Each connection will be contained and less likely to interfere with other connections.
- Security - Each thread can have different security attributes.
- Multiple Processors - Threading automatically will take advantage of multiple processors.
- No Serialization - Threading provides true concurrency. Without threading all requests must be handled by a single thread. For this to work each task to be performed must be broken up into small pieces that can always execute quickly. If any task part blocks or takes time to execute all other task parts will be put on hold until it is complete. After each task part is complete, the next one is processed, etc. With threading, each task can be programmed as a complete task and the operating system will divide CPU time among the tasks.
Thread Pooling
The creation and destruction of threads can be resource intensive. This is especially evident with servers that have short-lived connections. Such servers create a thread use it for very brief time and then destroy it. This causes for a very high frequency of creation and destruction of threads. An example of this is a time or even and web server. A single request is sent, and a simple answer returned. When using a browser to browse a web site hundreds of connections and disconnections to the server may occur.
Thread pooling can alleviate such situations. Instead of creating and destroying threads on demand, threads are borrowed from a list of inactive but already created list (pool). When a thread is no longer needed it is redeposited into the pool instead of being destroyed. While threads are in the pool they are marked inactive and thus do not consume CPU cycles. For a further improvement, the size of the pool can be adjusted dynamically to meet the current needs of the system.
Indy does support thread pooling. Thread pooling in Indy can be achieved via use of the TIdThreadMgrPool component.
Hundreds of Threads
With a busy server hundreds or even thousands of threads can easily be needed. There is a common misconception that hundreds of threads will instantly kill your system. This is a false belief.
With most servers threads spend most of their time waiting on data. While waiting on blocking calls the thread will be inactive. Thus in a server with 500 threads only 50 may be active at a single time.
The number of threads that your system has running now may surprise you. With only minimal services started and the following applications running:
My system has 333 threads currently created:
Even with 333 threads you can see that my CPU utilization is only at 1%. A heavily used IIS (Microsoft Internet Information Server) will create hundreds or thousands more threads.
Threads and Global Sections
Whenever multiple threads need to access data in a read/write fashion they must control access to the data to protect its integrity. This can be intimidating to programmers new to threading. However most servers do not require global data. Most servers perform compartmentalized functions. That is each thread performs an isolated task. Global read/write sections are an issue for many multi threaded applications, but global read/write sections typically are not an issue for most socket servers.
Indy Methodology
Indy is different than other Winsock components you may be familiar with. If you''''ve worked with other components, the best approach for you may be to forget how they work. Nearly all other components use non-blocking (asynchronous) calls and act asynchronously. They require you to respond to events, set up state machines, and often perform wait loops.
For example, with other components, when you call connect you must wait for a connect event to fire, or loop until a property indicates that you are connected. With Indy you merely call Connect, and wait for it to return. If it succeeds, it will return when it does. If it fails, it will raise an exception. Working with Indy is very much like working with files. Indy allows you to put all your code in one place, instead of scattered throughout different events. In addition, Indy is much easier and more suited to threading.
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