GP数据库登陆提示too many sessions

GP数据库psql登陆时提示如下信息：
Too many sessions
Could not acquire resources for additional sessions
Disconnect some sessions and try again

1.刚开始杀掉部分连接能登陆数据库，gpstate -e检查了当时数据库所有节点运行正常。
2.ps -ef|grep -i postgres|wc-l统计了当时master和segment所有节点的连接数均未超过max_connections参数配置的值
3.日志里面也没查询到明确的报错提示信息

有哪位大神遇到过这个问题？求解答！

疑问：GP连接报错中，too many clients,too many connections,too many sessions这三个有什么区别？