Node.js servers are very efficient concerning I/O and large number of client connection. But why is node.js not suitable for heavy CPU apps in comparison to a traditional multithreading server?
I read it here Felix Baumgarten
Node is, despite its asynchronous event model, by nature single threaded. When you launch a Node process, you are running a single process with a single thread on a single core. So your code will not be executed in parallel, only I/O operations are parallel because they are executed asynchronous. As such, long running CPU tasks will block the whole server and are usually a bad idea.
Given that you just start a Node process like that, it is possible to have multiple Node processes running in parallel though. That way you could still benefit from your multithreading architecture, although a single Node process does not. You would just need to have some load balancer in front that distributes requests along all your Node processes.
Another option would be to have the CPU work in separate processes and make Node interact with those instead of doing the work itself.
Related things to read:
A simple Node.js server is single-threaded, meaning that any operation that takes a long time to execute will block the rest of your program from running. Node.js apps manage to maintain a high level of concurrency by working as a series of events. When an event handler is waiting for something to happen (such as reading from the database), it tells Node to go ahead and process another event in the meantime. But since a single thread can only execute one instruction at a time, this approach can't save you from a function that needs to keep actively executing for a long time. In a multithreaded architecture, even if one function takes a long time to compute the result, other threads can still process other requests — and as long as you have a core that is not fully used at the time, there's a good chance they can do it about as quickly as if no other requests were running at all.
In order to deal with this, production Node.js apps that expect to hog a lot of CPU will usually be run in clusters. This means that instead of having several threads in one program's memory space, you run several instances of the same program under the control of one "master" instance. Each process is single-threaded, but since you have several of them, you end up gaining the benefits of multiple threads.