Mysql keep shutdown after few restart on fresh Centos installation

I have restart a few times - systemctl stop mysql and systemctl start mysql on my Centos 7. After some time, all my application on this machine will face this error

SQLSTATE[HY000] [2002] Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (111 "Connection refused")

when I check the status it seems fine

[$]systemctl status mysql
mysql.service - LSB: start and stop MySQL
   Loaded: loaded (/etc/rc.d/init.d/mysql)
   Active: active (exited) since Thu 2015-01-29 21:46:25 EST; 1 day 14h ago

Jan 29 21:46:24 xxx systemd[1]: Starting LSB: start and stop MySQL...
Jan 29 21:46:25 xxx mysql[13121]: Starting MySQL. SUCCESS!
Jan 29 21:46:25 xxx systemd[1]: Started LSB: start and stop MySQL.
Jan 31 11:49:59 xxx systemd[1]: Started LSB: start and stop MySQL.

but when try to login using mysql -u root -p the same error will come out. Is there any setting to configure after installation of MariaDB 10.0.16. Let me know what to tune for application like WordPress. For your information I only have 512MB of RAM and total of 20GB harddisk

thanks