How to prevent HTML5 Web Workers from locking up thus correctly responding to messages from parent

Question

I'm using web workers to do some CPU intensive work but have the requirement that the worker will respond to messages from the parent script while the worker is still processing.

The worker however will not respond to messages while it is locked in a processing loop, and I have not found a way to say poll the message queue. Thus it seems like the only solution is to break processing at an interval to allow any messages in the queue to be serviced.

The obvious options are to use a timer (say with setInterval) however I have read that the minimum delay between firings is quite long (http://ajaxian.com/archives/settimeout-delay) which is unfortunate as it will slow down processing alot.

What are other peoples thoughts on this? I'm going to try have the worker dispatch onmessage to itself at the end of each onmessage, thus effectively implementing one step of the processing loop per event received from itself, but just wanted to see if anyone had any ideas about this.

Thanks,

Answer 1

A worker can spawn sub workers. You can have your main worker act as your message queue, and when it receives a request for a long running operation, spawn a sub worker to process that data. The sub worker can then send the results back to the main worker to remove the event from the queue and return the results to the main thread. That way your main worker will always be free to listen for new messages and you have complete control over the queue.

--Nick

Answer 2

I ran into this issue myself when playing with workers for the first time. I also debated using setInterval, but I felt that this would be a rather hacky approach to the problem (and I had already went this way for my emulated multithreading). Instead, I settled on terminating the workers from the main thread (worker.terminate()) and recreating them if the task that they are involved in needs to be interrupted. Garbage collection etc seemed to be handled in my testing.

If there is data from these tasks that you want to save, you can always post it back to the main thread for storage at regular intervals, and if there is some logic you wish to implement regarding whether they are terminated or not, you can post the relevant data back at regular enough intervals to allow it.

Spawning subworkers would lead to the same set of issues anyway; you'd still have to terminate the subworkers (or create new ones) according to some logic, and I'm not sure it's as well supported (on chrome for example).

James