[PIG-2829] Use partial aggregation more aggresively

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 0.10.0
• Assignee: Unassigned
• Reporter: Jie Li
• Created: 19/Jul/12 22:34
• Updated: 06/Sep/12 05:08

Description

Partial aggregation (Hash Aggregation, aka in-map combiner) is a new feature in Pig 0.10 that will perform aggregation within map function. The main advantage against combiner is it avoids de/serializing and sorting the data, and it can auto disable itself if the data reduction rate is low. Currently it's disabled by default.

To leverage the power of PartialAgg more aggressively, several things need to be revisited:

1. The threshold of auto-disabling. Currently each mapper looks at first 1k (hard-coded) records to see if there's enough data size reduction (defaults to 10x, configurable). The check would happen earlier if the hash table gets full before processing the 1k records (hash table size is controlled by pig.cachedbag.memusage). We might want to relax these thresholds.

2. Dependency on the combiner. Currently the PartialAgg won't work without a combiner following it, so we need to provide separate options to enable each independently.

Issue Links

• is related to: PIG-2228 support partial aggregation in map task