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Simple description of worker and I/O threads in .NET

Asked 2010-01-20T08:29:06.323
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It's very hard to find detailed but simple description of worker and I/O threads in .NET

What's clear to me regarding this topic (but may not be technically precise):

  • Worker threads are threads that should employ CPU for their work;
  • I/O threads (also called "completion port threads") should employ device drivers for their work and essentially "do nothing", only monitor the completion of non-CPU operations.

What is not clear:

  • Although method ThreadPool.GetAvailableThreads returns number of available threads of both types, it seems there is no public API to schedule work for I/O thread. You can only manually create worker thread in .NET?
  • It seems that single I/O thread can monitor multiple I/O operations. Is it true? If so, why ThreadPool has so many available I/O threads by default?
  • In some texts I read that callback, triggered after I/O operation completion is performed by I/O thread. Is it true? Isn’t this a job for worker thread, considering that this callback is CPU operation?
  • To be more specific – do ASP.NET asynchronous pages user I/O threads? What exactly is performance benefit in switching I/O work to separate thread instead of increasing maximum number of worker threads? Is it because single I/O thread does monitor multiple operations? Or Windows does more efficient context switching when using I/O threads?
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Simply put a worker thread is meant to perform a short period of work and will delete itself when it has completed it. A callback may be used to notify the parent process that it has completed or to pass back data.

An I/O thread will perform the same operation or series of operations continuously until stopped by the parent process. It is so called because it typically device drivers run continuously monitor the device port. An I/O thread will typically create Events whenever it wishes to communicate to other threads.

All processes run as threads. Your application runs as a thread. Any thread may spawn worker threads or I/O threads (as you call them).

There is always a fine balance between performance and the number or type of threads used. Too many callbacks or Events handled by a process will severely degrade its performance due to the number of interruptions to its main process loop as it handles them.

Examples of a worker thread would be to add data into a database after user interaction or to perform a long mathematical calculation or write data to a file. By using a worker thread you free up the main application, this is most useful for GUIs as it doesn't freeze whilst the task is being performed.

answered 2010-01-20T08:49:38.180

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