MySQL/MariaDB crashes with error: Fatal error: cannot allocate memory for the buffer pool

Symptoms

1. Plesk is not accessible: 

   Error 2002. ERROR: PleskDBException: Unable to connect to database: mysql_connect(): Connection refused /var/run/mysqld/mysqld.sock (Error code: 2002)

2. MySQL/MariaDB crashes intermittently. There is an error in
/var/log/mysqld.log
file:

   InnoDB: Initializing buffer pool, size = 128.0M
   InnoDB: mmap(137363456 bytes) failed; errno 12
   InnoDB: Completed initialization of buffer pool
   InnoDB: Fatal error: cannot allocate memory for the buffer pool
   [ERROR] Plugin 'InnoDB' init function returned error.
   [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
   [ERROR] Unknown/unsupported storage engine: InnoDB
   [ERROR] Aborting

Cause

MySQL/MariaDB buffer size is bigger than free RAM available on the server.

Resolution

Contact system administrator to allocate more RAM to the server.

As a temporary workaround:

1. Connect to the server using SSH.
2. Check how much free RAM there is on the server:

   # free -hm
   total used free shared buffers cached
   Mem: 1.0G  122M 901M 92K 0B 61M
   -/+ buffers/cache: 61M 962M
   Swap: 1.0G 0B 1.0G

3. Set MySQL/MariaDB buffer pool size in
/etc/my.cnf less than amount of free RAM:

   innodb_buffer_pool_size=100M

4. Restart MySQL/MariaDB service:

   # service mysqld restart || service mysql restart || service mariadb restart