multi threading a web application

Question

I know there are many cases when it's best to multi thread an application but when is it best to multi thread a .net web Application?

Answer 1

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).

Answer 2

In order to benefit from multithreading your application has to do a significant amount of work that can be run in parallel. If this is not the case, the overhead of multithreading may very well top the benefits.

In my experience most web applications consist of a number of short running methods, so apart from the parallelism already offered by the hosting environment, I would say that it is rare to benefit from multithreading within the individual parts of a web application. There are probably examples where it will offer a benefit, but my guess is that it isn't very common.

Answer 3

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

Answer 4

ASP.NET is already capable of spawning several threads for processing several requests in parallel, so for simple request processing there is rarely a case when you would need to manually spawn another thread. However, there are a few uncommon scenarios that I have come across which warranted the creation of another thread:

• If there is some operation that might take a while and can run in parallel with the rest of the page processing, you might spawn a secondary thread there. For example, if there was a webservice that you had to poll as a result of the request, you might spawn another thread in Page_Init, and check for results in Page_PreRender (waiting if necessary). Though it's still a question if this would be a performance benefit or not - spawning a thread isn't cheap and the time between a typical Page_Init and Page_Prerender is measured in milliseconds anyway. Keeping a thread pool for this might be a little bit more efficient, and ASP.NET also has something called "asynchronous pages" that might be even better suited for this need.
• If there is a pool of resources that you wish to clean up periodically. For example, imagine that you are using some weird DBMS that comes with limited .NET bindings, but there is no pooling support (this was my case). In that case you might want to implement the DB connection pool yourself, and this would necessitate a "cleaner thread" which would wake up, say, once a minute and check if there are connections that have not been used for a long while (and thus can be closed off).

Another thing to keep in mind when implementing your own threads in ASP.NET - ASP.NET likes to kill off its processes if they have been inactive for a while. Thus you should not rely on your thread staying alive forever. It might get terminated at any moment and you better be ready for it.

Answer 5

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.