[HDFS-8988] Use LightWeightHashSet instead of LightWeightLinkedSet in BlockManager#excessReplicateMap

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.8.0, 3.0.0-alpha1
• Target Version/s: 2.8.0
• Hadoop Flags: Reviewed
• Assignee: Yi Liu
• Reporter: Yi Liu
• Created: 28/Aug/15 06:31
• Updated: 30/Aug/16 01:28
• Resolved: 12/Oct/15 07:13

Description

public final Map<String, LightWeightLinkedSet<Block>> excessReplicateMap = new HashMap<>();

LightWeightLinkedSet extends LightWeightHashSet and in addition it stores elements in double linked list to ensure ordered traversal. So it requires more memory for each entry (2 references = 8 + 8 bytes = 16 bytes, assume 64-bits system/JVM).
I have traversed the source code, and we don't need ordered traversal for excess replicated blocks, so could use LightWeightHashSet to save memory.