session配置为memcached或redis后的错误提示！！

happynetlife

1. $config['sess_driver'] = 'redis';
   
2. $config['sess_save_path'] = 'localhost:11211';
   
3. $config['sess_save_path'] = 'tcp://127.0.0.1:6379';

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配置后运行日志有大量如下错误：

1. ERROR - 2018-01-26 13:12:10 --> Session: Error while trying to obtain lock for ci_session:8gjd1nduds8ablv48m2m2ceeujlbp3je
   
2. ERROR - 2018-01-26 13:12:10 --> Severity: Warning --> Unknown: Failed to write session data (user). Please verify that the current setting of session.save_path is correct (tcp://127.0.0.1:6379) Unknown 0

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修改配置为memcached后也有类似提示，有没有人知道什么原因?

我运行环境是Centos7+nginx+php5.6，redis和memcached都是新的！CI3.1.6