Why node.js is fast when it's single threaded?

Question

Despite being single threaded, how is node.js is faster? I haven\'t run any tests to find statistics, but while digging in the node.js forums, I find everyone says it\'s fa

Answer 1

Because nodejs won't wait for the response, instead it follows event-driven programming with callbacks i.e Once a request is submitted it will be pushed in an event queue and each request is handled by a single thread but this thread will just submit the request and move to the next request and so on and never waits for the response. Once the request is processed the respective callback function of the request will be executed.

Answer 2

Node.js is not single threaded : see https://nodejs.org/about/ :

any connections can be handled concurrently

In fact, it does not use system threads, but instead uses the V8 engine along with the libuv library for multi-threading through asynchronous callbacks.

Also, you can use additional child process through child_process.fork

Finally, this does not in any way condition speed of the response or overall speed of the engine. Multi-threading is here for scalability.

Answer 3

First, why is a program faster when multi-threaded ?

It's partly due to the fact a multi-threaded program can run on multiple cores but the main reason, by far, is that when a thread is waiting for some IO operation (which is very often, especially in a server), the other threads can still progress.

Now, what about node ?

Node isn't single threaded. The user script in JS is executed in one thread but all IO operations are natively handled by libuv and the OS which are multi-threaded.

More explanation here.

In practice, this means that several requests are handled in parallel. Here's a very (very) simplified example of a possible sequence of actions:

user script                     | node + OS "threads" (libuv)
-------------------------------------------------------------
receive and analyze request 1   |
ask node for file 1             | fetching file 1
receive and analyze request 2   | fetching file 1
ask node for file 2             | fetching file 1, fetching file 2
prepare response header 1       | fetching file 2
tell node to send file 1        | send file 1, fetching file 2
prepare response header 2       | send file 1
tell node to send file 2        | send file 1, send file 2

The whole architecture of node (and io.js) makes it simple to have a high level of parallelism. The user thread is only called by the event loop for very short tasks which stop at the next IO operation (well, not really only IO, but most often) when your code gives to node a callback that will be called when the operation finished.

Of course this only works when you're using the asynchronous functions of Node. Any time you use a function ending in "Sync" like writeFileSync, you're defeating the concurrency.