Increasing Systemd Thread Limits
In some high workload, and large scale multi-database environments, you may find that your Systemd unit configuration limits the maximum number of processes ("tasks") too low for your use case.
When this problem occurs, you can see the following in the Neo4j
java.lang.OutOfMemoryError: unable to create native thread: possibly out of memory or process/resource limits reached
If you are looking at the output from
systemctl status neo4j you will see the number of running tasks is at or near to the limit, 4915 in this example:
● neo4j.service - Neo4j Graph Database Loaded: loaded (/lib/systemd/system/neo4j.service; enabled; vendor preset: enabled) Active: active (running) since Thu 1970-01-01 00:00:00 UTC; 48min ago Main PID: 20000 (java) Tasks: 4915 (limit: 4915) CGroup: /system.slice/neo4j.service ├─20000 /usr/bin/java org.neo4j.server.startup.Neo4jCommand console └─20047 /usr/lib/jvm/java-11-openjdk-amd64/bin/java com.neo4j.server.enterprise.EnterpriseEntryPoint
Looking further, you might find this reported in your Linux
messages file as something similar to:
Jan 01 00:48:00 neo4j-core1 kernel: [ TIME ] cgroup: fork rejected by pids controller in /system.slice/neo4j.service Jan 01 00:48:02 neo4j-core1 neo4j[ PID ]: [ TIME ][warning][os,thread] Failed to start thread - pthread_create failed (EAGAIN) for attributes: stacksize: 1024k, guardsize: 0k, detached. Jan 01 00:48:02 neo4j-core1 neo4j[ PID ]: Exception in thread "neo4j.Scheduler-1" java.lang.OutOfMemoryError: unable to create native thread: possibly out of memory or process/resource limits reached
Systemd may then terminate and restart Neo4j Server.
In these cases, you may want to conservatively increase the maximum thread limit as a workaround. This should only need to be temporary in all but the most extreme cases.
First, ensure Neo4j is stopped:
systemctl stop neo4j
Then edit the
neo4j.service unit file:
systemctl edit neo4j
and add/increase the
[Service] # The user and group which the service runs as. User=neo4j Group=neo4j # If it takes longer than this then the shutdown is considered to have failed. # This may need to be increased if the system serves long-running transactions. TimeoutSec=120 # Increase the systemd process / task limit TasksMax=6500
After uncommenting the above lines, restart neo4j.
systemctl daemon-reload systemctl start neo4j
Again, be very conservative when increasing the task limit for a system service. High process thread counts can be a strong indicator of poor configuration, sizing issues, or plugins that are misbehaving.
As with all workarounds, they should be used sparingly until a root cause can be fully identified and a more permanent solution put in place.
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