multithreadinglanguage-agnostic

When is multi-threading not a good idea?

I was recently working on an application that sent and received messages over Ethernet and Serial. I was then tasked to add the monitoring of DIO discretes. I throught,

"No reason to interrupt the main thread which is involved in message processing, I'll just create another thread that monitors DIO."

This decision, however, proved to be poor. Sometimes the main thread would be interrupted between a Send and a Receive serial message. This interruption would disrupt the timing and alas, messages would be lost (forever).

I found another way to monitor the DIO without using another thread and Ethernet and Serial communication were restored to their correct functionality.

The whole fiasco, however, got me thinking. Are their any general guidelines about when not to use multiple-threads and/or does anyone have anymore examples of situations when using multiple-threads is not a good idea?

**EDIT:Based on your comments and after scowering the internet for information, I have composed a blog post entitled When is multi-threading not a good idea?

Solution
    1. On a single processor machine and a desktop application, you use multi threads so you don't freeze the app but for nothing else really.
    2. On a single processor server and a web based app, no need for multi threading because the web server handles most of it.
    3. On a multi-processor machine and desktop app, you are suggested to use multi threads and parallel programming. Make as many threads as there are processors.
    4. On a multi-processor server and a web based app, no need again for multi threads because the web server handles it.

    In total, if you use multiple threads for other than un-freezing desktop apps and any other generic answer, you will make the app slower if you have a single core machine due to the threads interrupting each other.

    Why? Because of the hardware switches. It takes time for the hardware to switch between threads in total. On a multi-core box, go ahead and use 1 thread for each core and you will greatly see a ramp up.