inotify

inotify instance limit reached (Ubuntu)

Problem

An application fails with an error like this:

System.IO.IOException: The configured user limit (128) on the number of inotify instances has been reached, or the per-process limit on the number of open file descriptors has been reached.

inotify is the kernel API that applications use to watch files and directories for changes. Each user has two caps:

• fs.inotify.max_user_instances: how many inotify instances one user can open. The error above shows the default of 128.
• fs.inotify.max_user_watches: how many paths the user can watch in total.

Development tools that watch many files (.NET file watchers, IDEs, bundlers) hit these caps quickly.

Check current values

sudo sysctl -a | grep inotify

Temporary fix (lasts until reboot)

sudo sysctl -w fs.inotify.max_user_instances=512
sudo sysctl -w fs.inotify.max_user_watches=524288

Permanent fix

Put the same settings in a drop-in file under /etc/sysctl.d/ and reload:

sudo tee /etc/sysctl.d/99-inotify.conf > /dev/null <<'EOF'
fs.inotify.max_user_instances=512
fs.inotify.max_user_watches=524288
EOF
sudo sysctl --system

Verify the live values:

sysctl fs.inotify.max_user_instances fs.inotify.max_user_watches

Why

• max_user_instances=512 raises the per-user instance cap from the default of 128, so several watcher-based tools can run at once without hitting the error.
• max_user_watches=524288 gives a large watch budget. Values in the hundreds are far too low for a real project tree. Watches only use kernel memory once they are added.
• A drop-in file in /etc/sysctl.d/ keeps the change separate from the distribution-managed /etc/sysctl.conf. Persist the same two values you tested with the temporary fix, so the setting survives reboots.