ThreadWeaver

Use Cases

ThreadWeaver provides a solution to a number (but not all) multithreading problems. Let's have a look:

  • How do you implement a single operation that takes a lot of CPU power, but is hard to handle in chunks (Example: scale an image) when you want your GUI to remain responsive? Encapsulate the operation in a class object derived from Job instead of a method and put it in the static instance (Weaver::instance). Connect to the Job's done() signal to receive a notification when it is completed.
  • How do you implement a CPU intensive operation while time-critical operations are executed (load and decode an MP3 while another one is fed to the audio device): Implement both the file loading operation and the play operation in a job and queue both at the same time. There is also a synchronous sleep method in the Job class if a job needs to be delayed for a number of milliseconds after it is taken over by a thread.
  • How do you implement many small operations where the cost of each individual one is hard to estimate (loading two hundred icons from an NFS drive): Create a common or a number of specialized job classes. Queue all of them. Either connect to the individual done signal to process every item when it has been finished or connect to Weaver::jobsDone() to receive a notification when all of the jobs are done.
  • How do you implement an operation with complex control flow and dependencies in the execution order (load, parse and display an HTML document with embedded media): Create jobs for the individual steps you need to perform. Try to split the whole operation in as many independent, parallelizable parts as possible. Now declare the execution dependencies. A job will only be executed when all jobs it depends on are finished. This way every individual operation will be executed as soon as it becomes possible. Connect to the final jobs done() signal to be notified when all parts of the whole operations have been executed. If necessary (if there is more than one final object), create a dummy job object that depends on all of them to have one central end point of execution.

As you can see, ThreadWeaver can provide solutions for simple, but also for complex cases. For an example on how job dependencies can be modeled and used to create elegant, streamlined solutions for control flow modeling, see the Simple Multithreaded Image Viewer (SMIV) example in the Tests directory.

ThreadWeaver focuses on operations that can be implemented in Jobs. To create solutions where a thread is supposed to constantly run to perform an ongoing operation that, for example, spans the whole application live, it has to be verified that it is the right approach. It is possible to add very long or neverending operations to the queue, but it will constantly bind a thread to that operation. It might still make sense to use ThreadWeaver, as the creation, handling and destruction of the threads is already taken care of, but the minimum inventory size (thread count) should be increased accordingly to provide for enough threads to process the other jobs.

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