[HIVE-6518] Add a GC canary to the VectorGroupByOperator to flush whenever a GC is triggered

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 0.13.0
• Fix Version/s: 0.13.0
• Component/s: Query Processor
• Release Note: Flush VectorGroupBy aggregation hashes in case of a full GC
• Assignee: Gopal Vijayaraghavan
• Reporter: Gopal Vijayaraghavan
• Created: 27/Feb/14 23:16
• Updated: 17/Mar/14 18:41
• Resolved: 17/Mar/14 16:42

Description

The current VectorGroupByOperator implementation flushes the in-memory hashes when the maximum entries or fraction of memory is hit.

This works for most cases, but there are some corner cases where we hit GC ovehead limits or heap size limits before either of those conditions are reached due to the rest of the pipeline.

This patch adds a SoftReference as a GC canary. If the soft reference is dead, then a full GC pass happened sometime in the near past & the aggregation hashtables should be flushed immediately before another full GC is triggered.