[IMPALA-7637] Include more hash table stats in profile

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: Impala 3.4.0
• Component/s: Backend
• Labels: observability ramp-up
• Epic Link: Improve Impala Profile User Experience
• Target Version: Product Backlog
• Epic Color: ghx-label-4
• Assignee: Yongzhi Chen
• Reporter: Tim Armstrong
• Created: 27/Sep/18 00:31
• Updated: 03/Oct/19 13:21
• Resolved: 03/Oct/19 13:21

Description

Our hash table collects some useful stats about collisions and travel length, but then we don't do anything to expose them: https://github.com/apache/impala/blob/540611e863fe99b3d3ae35f8b94a745a68b9eba2/be/src/exec/hash-table.h#L989

We should add some of them to the profile, maybe:

• the number of probes
• the average travel length per probe
• the number of hash collisions
• (optional) the number of hash table resizes. We already have the hash table size and the resize time, which I think is sufficient to debug most problems with resizes.