legate.core.Runtime.issue_mapping_fence

Runtime.issue_mapping_fence(self) → void

Issue a mapping fence.

A mapping fence, when issued, blocks mapping of all downstream operations before those preceding the fence get mapped. An issue_mapping_fence() call returns immediately after the request is submitted to the runtime, and the fence asynchronously goes through the runtime analysis pipeline just like any other Legate operations. The call also flushes the scheduling window for batched execution.

Mapping fences only affect how the operations are mapped and do not change their execution order, so they are semantically no-op. Nevertheless, they are sometimes useful when the user wants to control how the resource is consumed by independent tasks. Consider a program with two independent tasks A and B, both of which discard their stores right after their execution. If the stores are too big to be allocated all at once, mapping A and B in parallel (which can happen because A and B are independent and thus nothing stops them from getting mapped concurrently) can lead to a failure. If a mapping fence exists between the two, the runtime serializes their mapping and can reclaim the memory space from stores that would be discarded after A’s execution to create allocations for B.