multi threading a web application

A web application is almost certainly already multi threaded by the hosting environment (IIS etc). If your page is CPU-bound (and want to use multiple cores), then arguably multiple threads is a bad idea, as when your system is under load you are already using them.

The time it might help is when you are IO bound; for example, you have a web-page that needs to talk to 3 external web-services, talk to a database, and write a file (all unrelated). You can do those in parallel on different threads (ideally using the inbuilt async operations, to maximise completion-port usage) to reduce the overall processing time - all without impacting local CPU overly much (here the real delay is on the network).

Of course, in such cases you might also do better by simply queuing the work in the web application, and having a separate service dequeue and process them - but then you can't provide an immediate response to the caller (they'd need to check back later to verify completion etc).

Multithreading is a technique to provide a single process with more processing time to allow it to run faster. It has more threads thus it eats more CPU cycles. (From multiple CPU's, if you have any.) For a desktop application, this makes a lot of sense. But granting more CPU cycles to a web user would take away the same cycles from the 99 other users who are doing requests at the same time! So technically, it's a bad thing.

However, a web application might use other services and processes that are using multiple threads. Databases, for example, won't create a separate thread for every user that connects to them. They limit the number of threads to just a few, adding connections to a connection pool for faster usage. As long as there are connections available or pooled, the user will have database access. When the database runs out of connections, the user will have to wait.

So, basically, the use of multiple threads could be used for web applications to reduce the number of active users at a specific moment! It allows the system to share resources with multiple users without overloading the resource. Instead, users will just have to stand in line before it's their turn.

This would not be multi-threading in the web application itself, but multi-threading in a service that is consumed by the web application. In this case, it's used as a limitation by only allowing a small amount of threads to be active.

IMHO you should avoid the use of multithread in a web based application.

maybe a multithreaded application could increase the performance in a standard app (with the right design), but in a web application you may want to keep a high throughput instead of speed.

but if you have a few concurrent connection maybe you can use parallel thread without a global performance degradation