网站每隔12天就出现Error establishing a database connection

如题，刚建的站点，基本没人访问时，每隔1,2天就出现Error establishing a database connection，人稍微多点过一会也会出现Error establishing a database connection，麻烦大神指点什么原因。
数据库的错误日志

171115 14:14:30 mysqld_safe Number of processes running now: 0
171115 14:14:30 mysqld_safe mysqld restarted
171115 14:14:31 [Note] Plugin 'FEDERATED' is disabled.
171115 14:14:31 InnoDB: Mutexes and rw_locks use GCC atomic builtins
171115 14:14:31 InnoDB: Compressed tables use zlib 1.2.3
171115 14:14:31 InnoDB: Using Linux native AIO
171115 14:14:31 InnoDB: Initializing buffer pool, size = 128.0M
InnoDB: mmap(137363456 bytes) failed; errno 12
171115 14:14:31 InnoDB: Completed initialization of buffer pool
171115 14:14:31 InnoDB: Fatal error: cannot allocate memory for the buffer pool
171115 14:14:31 [ERROR] Plugin 'InnoDB' init function returned error.
171115 14:14:31 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
171115 14:14:31 [ERROR] Unknown/unsupported storage engine: InnoDB
171115 14:14:31 [ERROR] Aborting


已经调大了my.cnf中的
key_buffer_size = 400M
#innodb_buffer_pool_size = 400M
（服务器内存1G）
但是还是出现同样的问题。


详细的错误提示如下



Warning: mysqli_real_connect(): (HY000/2002): No such file or directory in /alidata/www/phpwind/wp-includes/wp-db.php on line 1548


Deprecated: mysql_connect(): The mysql extension is deprecated and will be removed in the future: use mysqli or PDO instead in /alidata/www/phpwind/wp-includes/wp-db.php on line 1578


Warning: mysql_connect(): No such file or directory in /alidata/www/phpwind/wp-includes/wp-db.php on line 1578


查看wp-db.php文件相应内容如文末




麻烦大神指点问题出现在哪？