Node.js: event loop lag / blocked synchronously

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

import { monitorEventLoopDelay } from 'perf_hooks';
const h = monitorEventLoopDelay({ resolution: 10 });
h.enable();
setInterval(() => console.log(h.percentile(99) / 1e6), 5000);
# Move CPU-heavy work to worker_threads
# Avoid sync crypto/fs and huge JSON.parse on hot paths

How to diagnose Performance errors

Performance failures rarely produce an error message. They produce timeouts elsewhere. The three classic causes are stop-the-world garbage collection, thread pool starvation (all workers blocked on I/O so new requests queue), and event loop blocking in single-threaded runtimes. All three look identical from outside: latency climbs, then upstream timeouts fire. Distinguishing them requires looking inside the process.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Measure the p99, not the mean. Averages hide exactly the pauses that cause timeouts.
2. For the JVM, enable GC logging (-Xlog:gc*) and correlate pause durations with latency spikes before tuning anything.
3. For thread pools, log active versus queued task counts. A queue that grows while CPU is idle is starvation, and the fix is asynchronous I/O, not more threads.
4. In Node.js, measure event loop lag directly (perf_hooks.monitorEventLoopDelay). Any synchronous work over a few milliseconds per request will show up here.
5. Profile before optimising. pprof, async-profiler, py-spy and Chrome's profiler all point at the real hot path, which is rarely where intuition suggests.

Tools worth reaching for

• async-profiler
• go tool pprof
• py-spy
• perf_hooks.monitorEventLoopDelay
• -Xlog:gc*

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 nodejs.org

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• java.lang.OutOfMemoryError: GC overhead limit exceededThe JVM spent over 98 percent of recent time collecting and recovered less than 2 percent of…
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• Node.js: high latency from synchronous crypto or JSON on the event loopNode runs JavaScript on a single thread. A synchronous bcrypt hash, a large JSON.parse, or a…
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See all 6 Performance errors →

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Something missing or wrong?

This entry is maintained by hand. If the fix is out of date, incomplete, or you have a better one, email a correction and it will be reviewed.