Recommendation on table_open_cache_instances

[lchenay]

table_open_cache could be a source of contention as default value is 1 in mysql version prior to 7.

The recommended value is 0.5 per CPU to split work load across CPU and reduce locking issue on the table cache partition.
Using a big value doesn't reduce performance.
Mysql 7 have a default value at 16.

[grooverdan]

Note mariadb-10.2.2+ autosize this and it has a different meaning to MySQL https://mariadb.com/kb/en/server-system-variables/#table_open_cache_instances