Why Node.js spins 7 threads per process

Question

When a Node.js process is spun up top command shows 7 threads attached to the process. What are all these threads doing? Also, as the load on the API increases, with the request handlers themselves asynchronously awaiting other upstream API calls does Node spawn additional worker threads? I see in top that it does that. But I was thinking this only happens for file I/o. Why does it need these additional worker threads?

Answer 1

LIBUV (the underlying cross platform system library that node.js is built-on) uses a thread pool for certain operations such as disk I/O and some crypto operations. By default that thread pool contains 4 threads.

Plus, there is a thread for the execution of your Javascript so that accounts for 5.

Then, it appears there is a thread used by the garbage collector for background marking of objects (per this reference from a V8 developer ) and this article . That would make 6.

I don't know for sure what the 7th one would be. It's possible there's a thread used by the event loop itself.

Then, starting some time around 2018, it appears that nodejs switched to a separate set of threads for DNS requests (separate from the file I/O thread pool). This was probably because of problems in node.js where 4 slow DNS requests could block all file I/O because they took over the thread pool. So, now it looks like node.js used the C-ARES library for DNS which makes its own set of threads.

FYI, you can actually control the thread pool size with the UV_THREADPOOL_SIZE environment variable.

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And, of course, you can create your own Worker Threads that actually create new instances of the V8 Javascript execution engine (so they will likely end up creating more than one new thread).