how to enalbe MySQL 8 innodb_dedicated_server option Initially, I was looking for tuning the MySql 8 instance then I found that there is something like autoconfiguration feature added with the 8th version. Then I visited the official MySQL website for documentation and found this but was unable to wrap my head around this.

As given in the official documentation when this option is enabled MySql autoconfigure certain cache and butter options so that I don't have to change the config when Scale the Server Instance.

When I tried to add innodb_dedicated_server = ON to my.cnf and mysqld.cnf, it 
resulted in being unable to start the server.

got nothing in /var/log/mysql/error.log
and found following via journalctl -xe

Jan 08 17:41:50 fa-b2b-mysql mysql-systemd-start[31493]: Once the problem is resolved, restart the service.
Jan 08 17:41:50 fa-b2b-mysql systemd[1]: mysql.service: Control process exited, code=exited status=1
Jan 08 17:41:50 fa-b2b-mysql systemd[1]: mysql.service: Failed with result 'exit-code'.
Jan 08 17:41:50 fa-b2b-mysql systemd[1]: Failed to start MySQL Community Server.
-- Subject: Unit mysql.service has failed
-- Defined-By: systemd
-- Support: http://www.ubuntu.com/support
-- 
-- Unit mysql.service has failed.
-- 
-- The result is RESULT.
Jan 08 17:41:50 fa-b2b-mysql systemd[1]: mysql.service: Service hold-off time over, scheduling restart.
Jan 08 17:41:50 fa-b2b-mysql systemd[1]: mysql.service: Scheduled restart job, restart counter is at 5.
-- Subject: Automatic restarting of a unit has been scheduled
-- Defined-By: systemd
-- Support: http://www.ubuntu.com/support
-- 
-- Automatic restarting of the unit mysql.service has been scheduled, as the result for
-- the configured Restart= setting for the unit.
Jan 08 17:41:50 fa-b2b-mysql systemd[1]: Stopped MySQL Community Server.
-- Subject: Unit mysql.service has finished shutting down
-- Defined-By: systemd
-- Support: http://www.ubuntu.com/support
-- 
-- Unit mysql.service has finished shutting down.
Jan 08 17:41:50 fa-b2b-mysql systemd[1]: mysql.service: Start request repeated too quickly.
Jan 08 17:41:50 fa-b2b-mysql systemd[1]: mysql.service: Failed with result 'exit-code'.
Jan 08 17:41:50 fa-b2b-mysql systemd[1]: Failed to start MySQL Community Server.
-- Subject: Unit mysql.service has failed
-- Defined-By: systemd
-- Support: http://www.ubuntu.com/support
-- 
-- Unit mysql.service has failed.
-- 
-- The result is RESULT.
(I wrote a bug report about that doc page.)

You do not have any other apps (Apache, Java, etc) running on the machine, correct?

Plan A:  Manually with innodb_buffer_pool_size = 1200M, I think 2G is too big for 4G of RAM.

Plan B:  Write a bug report.  In it provide your entire my.cnf and the exact version you are using (bugs.mysql.com).  Then abandon innodb_dedicated_server = ON.