The described implementations relate to efficient scheduling of transactions and tasks. A memory location, address, or variable previously accessed by a blocked entity is observed periodically to determine an appropriate time to wake and retry the blocked entity. If the previous accessed memory location, address or variable changes state, a scheduler wakes the blocked entity and the blocked entity retries processing. A doubly-indexed data structure of blocked entities and memory locations associated with the blocked entities may be used to efficiently determine when a retrying execution would be profitable.