增加 Systemd 线程限制
问题
在一些高负载和大规模多数据库环境中，您可能会发现 Systemd 单元配置将最大进程（“任务”）数量限制得过低，无法满足您的使用场景。
当出现此问题时，您可以在 Neo4j debug.log 文件中看到如下信息
java.lang.OutOfMemoryError: unable to create native thread: possibly out of memory or process/resource limits reached
如果您查看 systemctl status neo4j 的输出，您会看到运行中的任务数已达到或接近限制，例如本例中的 4915
● 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
进一步查看时，您可能会在 Linux syslog 或 messages 文件中发现类似如下的报告
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 可能随后终止并重启 Neo4j Server。
变通方案
在这些情况下，您可能希望保守地提升最大线程限制作为变通方案。除极端情况外，这通常仅需临时使用。
首先，确保 Neo4j 已停止
systemctl stop neo4j
然后编辑 neo4j.service 单元文件
systemctl edit neo4j
并添加/增加 TasksMax 设置
[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取消上述行的注释后，重新启动 neo4j。
systemctl daemon-reload systemctl start neo4j
再次强调，在提升系统服务的任务限制时请非常保守。高进程线程计数可能是配置不当、规格不足或插件异常的强烈指示。
与所有变通方案一样，应在根本原因完全查明并制定更永久的解决方案之前，尽量少用。
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