When to use Multithread?

Question

When do you use threads in a application? For example, in simple CRUD operations, use of smtp, calling webservices that may take a few time if the server is facing bandwith issues, etc.

To be honest, i don't know how to determine if i need to use a thread (i know that it must be when we're excepting that a operation will take a few time to be done).

This may be a "noob" question but it'll be great if you share with me your experience in threads.

Thanks

Answer 1

I added C# and .NET tags to your question because you mention C# in your title. If that is not accurate, feel free to remove the tags.

There are different styles of multithreading. For example, there are asynchronous operations with callback functions. .NET 4 introduces the parallel Linq library. The style of multithreading you would use, or whether to use any at all, depends on what you are trying to accomplish.

Parallel execution, such as what parallel Linq would generally be trying to do, takes advantage of multiple processor cores executing instructions that do not need to wait for data from each other. There are many sources for such algorithms outside Linq, such as this. However, it is possible that parallel execution may be unable to you or that it does not suit your application.

More traditional multithreading takes advantage of threading within the .NET library (in this case) as provided by System.Thread. Remember that there is some overhead in starting processes on threads, so only use threads when the advantages of doing so outweigh this overhead. Generally speaking, you would only want to use this type of single-processor multithreading when the task running under the thread will have long gaps in which the processor could be doing something else. For example, I/O from hard disk (and, consequently, from a database system that uses one) is many orders of magnitude slower than memory access. Network access can also be slow, as another example. Multithreading could allow another process to be running while waiting for these slow (compared to the processor) operations to complete.

Another example when I have used traditional multithreading is to cache some values the first time a particular ASP.NET page is accessed within a session. I kick off a thread so that the user does not have to wait for the caching to complete before interacting with the page. I also regulate the behavior when the caching does not complete before the user requests another page so that, if the caching does not complete, it is not a problem. It simply makes some further requests faster that were previously too slow.

Consider also the cost that multithreading has to the maintainability of your application. Threaded applications can be harder to debug, for example.

I hope this answers your question at least somewhat.