Question about message loop

Question

I have a question haunting me for a long time.

Short version:

What's the working paradigm of Windows Message Loop?

Detailed version:

When we start a Windows application (not a console application), we can interact with it through mouse or keyboard. The application retrieve all kinds of messages representing our movements from its meesage queue. And it is Windows that is responsible for collecting our actions and properly feeding messages into this queue. But doesn't this scenario mean that Windows has to run infinitively?

I think the Windows scheduler should be running all the time. It could possibly be invoked by a time interrupt at a pre-defined interval. When the scheduler is trigged by the time interrupt, it swithes current thread for the next pending thread. A single thread can only get its message with GetMessage() when it is scheduled to run.

I am wondering if there's only one Windows application running, will this application got more chance to get its message?

Update - 1 (9:59 AM 11/22/2010)

Here is my latest finding:

According to < Windows via C/C++ 5th Edition > Chapter 7 Section: Thread Priorities

...For example, if your process' primary thread calls GetMessage() and the system sees that no messages are pending, the system suspends your porcess' thread, relinquishes the remainder of the thread's time slice, and immediately assigns the CPU to another waiting thread.

If no messages show up for GetMessage to retrieve, the process' primary thread stays suspended and is never assigned to a CPU. However, when a message is placed in the thread's queue, the system knows that the thread should no longer be suspended and assigns the thread to a CPU if no higher-priority threads need to execute.

My current understanding is:

In order for the system to know when a message is placed in a thread's queue, I can think of 2 possible approaches:

1 - Centralized approach: It is the system who is responsible to always check EVERY thread's queue. Even that thread is blocked for the lacking of messages. If any message is availabe, the system will change the state of that thread to schedulable. But this checking could be a real burden to the system in my opinion.

2 - Distributed approach: The system doesn't check every thread's queue. When a thread calls GetMessage and find that no message is available, the system will just change the thread's state to blocked, thus not schedulable any more. And in the future no matter who places a message into a blocked thread's queue, it is this "who"(not the system) that is responsible to change the the thread's state from blocked to ready (or whatever state). So this thread is dis-qualified for scheduling by the system and re-qualified by someone else in the regard of GetMessage. What the system cares is just to schedule the runable threads. The system doesn't care where these schedulable threads come from. This approach will avoid the burden in approach 1, and thus avoid the possible bottleneck.

In fact, the key point here is, how are the states of the threads changed? I am not sure if it is really a distributed paradigm as shown in appraoch 2, but could it be a good option?

Answer 1

Applications call GetMessage() in their message loop. If the message queue is empty, the process will just block until another message becomes available. Thus, GetMessage is a processes' way of telling Windows that it doesn't have anything to do at the moment.

Answer 2

I am wondering if there's only one Windows application running, will this application got more chance to get its message?

Well yeah probably, but I think you might be missing a crucial point. Extracting a message from the queue is a blocking call. The data structure used is usually referred to as a blocking queue. The dequeue operation is designed to voluntarily yield the current thread's execution if the queue is empty. Threads can stay parked using a various different methods, but it is likely that thread remains in a waiting state using kernel level mechanisms in this case. Once the signal is given that the queue has items available the thread may go into a ready state and the scheduler will start assigning its fair share of the CPU. In other words, if there are no messages pending for that application then it just sits there in an idle state consuming close to zero CPU time.

Answer 3

The fewer threads you have running (time slices are scheduled to threads, not processes), the more chances any single application will have to pull messages from its queue. Actually, this has nothing to do with Windows messages; it's true for all multithreading; the more threads of the same or higher priority which are running, the fewer time slices any thread will get.

Beyond that, I'm not sure what you are really asking, though...