MySQL/InnoDB defaults to REPEATABLE READ, I believe. We started running into locking/concurrency issues in this mode a long time ago and found that for our uses

MySQL/InnoDB defaults to REPEATABLE READ, I believe. We started running into locking/concurrency issues in this mode a long time ago and found that for our uses, 99.9% of the time READ COMMITTED would work just fine. We now use READ COMMITTED as the default and switch to REPEATABLE READ only when needed. We handle about 100x more traffic now than we did when we were forced to make the switch.