ReferenceGlossary
Event loop
A single thread that waits for I/O events and runs their callbacks one at a time. It serves many waiting connections cheaply, but a callback that uses the CPU for a long time blocks every other request on that loop.
Runtimes
In Quorum#
Node.js runs one event loop and uses one core; Node.js cluster runs one loop per core. A CPU-heavy request, such as an 80 ms password hash at login, stalls the requests behind it, and the stall lands on the p99. In the Event loop block Incident each login costs 4× the CPU. The libuv thread pool research or an Auth Provider takes the hash off the loop.
Read next#
Async I/O, Bcrypt, Green threads, Percentile (p50, p95, p99)