CodeParkUV_THREADPOOL_SIZE is an environment variable that sets the size of libuv's thread pool. libuv is the C library that gives Node its event loop and async I/O. The pool has 4 threads by default, and the whole process shares it. The pool runs this work:
- file system operations
dns.lookup- async crypto, such as
pbkdf2andscrypt zlibcompression
Change it when an app does a lot of this work at the same time. For example, the app hashes passwords with scrypt, reads files and resolves hostnames at once. Then requests wait in a queue behind each other, even though the event loop is idle. A bigger pool lets more of that work run in parallel. For example: UV_THREADPOOL_SIZE=16 node server.js.
Set it in the environment before the process starts, because Node creates the pool the first time it is used.
Two limits:
- More threads than CPU cores does not speed up CPU-bound work.
- It does nothing for network I/O, because network I/O never uses the pool.
- The default pool size is 4, and the whole process shares the pool.
- It affects fs,
dns.lookup, async crypto and zlib. - A full pool makes work wait in a queue, even when the event loop is idle.
- Set it before startup, because the pool is created lazily on first use.
- It has no effect on network sockets.