amazon ec2 - Mysql shutsdown in Centos -

i have installed centos on amazon ec2 .on centos installed apache , mysql . mysql server goes down giving message

 160128 17:26:45 [warning] ip address 'xx.xx.xx.xx' not resolved: name or service not known  160129 02:29:52 mysqld_safe number of processes running now: 0  160129 02:29:52 mysqld_safe mysqld restarted  160129  2:30:07 [note] /usr/libexec/mysql55/mysqld (mysqld 5.5.45) starting process 20398 ...  160129  2:30:07 [note] plugin 'federated' disabled.  160129  2:30:07 innodb: innodb memory heap disabled  160129  2:30:07 innodb: mutexes , rw_locks use gcc atomic builtins  160129  2:30:07 innodb: compressed tables use zlib 1.2.8  160129  2:30:07 innodb: using linux native aio  160129  2:30:10 innodb: initializing buffer pool, size = 128.0m  innodb: mmap(137363456 bytes) failed; errno 12  160129  2:30:10 innodb: completed initialization of buffer pool  160129  2:30:10 innodb: fatal error: cannot allocate memory buffer pool  160129  2:30:10 [error] plugin 'innodb' init function returned error.  160129  2:30:10 [error] plugin 'innodb' registration storage engine failed.  160129  2:30:10 [error] unknown/unsupported storage engine: innodb  160129  2:30:10 [error] aborting   160129  2:30:10 [note] /usr/libexec/mysql55/mysqld: shutdown complete   160129 02:30:10 mysqld_safe mysqld pid file /var/run/mysqld/mysqld.pid ended 

can tell me whats exact issue . thanks