We should avoid calling LlapTaskScheduler to get initial isguaranteed flag for all the tasks. It causes arbitrary delays in sending tasks out. Since communicator is a single thread, any blocking there delays all the tasks.
There are https://jira.apache.org/jira/browse/TEZ-4192 and https://jira.apache.org/jira/browse/HIVE-23589 for a proper solution to this. However, that requires a Tez release which seems far right now. We can replace the current hack with another hack that does not require locking.