Dear Tim,
I have tried your recommendation. The result is very great. Big sites should have this option disabled rather than enabled by default. And Moodle has given wrong advice about this.
Now, there is no table locking issue with mdl_cache_text as this option is disabled. However, there are still serious table locking issues with mdl_grade_item and mdl_forum_discussions. Is there any way to improve them as well? And does Moodle have any plan to improve them in the future?
Thanks a lot.