Yi Liu created HDFS-8859: ---------------------------- Summary: Improve DataNode (ReplicaMap) memory footprint to save about 45% Key: HDFS-8859 URL: https://issues.apache.org/jira/browse/HDFS-8859 Project: Hadoop HDFS Issue Type: Improvement Components: datanode Reporter: Yi Liu Assignee: Yi Liu
By using following approach we can save about *45%* memory footprint for each block replica in DataNode memory (This JIRA only talks about *ReplicaMap* in DataNode), the details are: In ReplicaMap, {code} private final Map<String, Map<Long, ReplicaInfo>> map = new HashMap<String, Map<Long, ReplicaInfo>>(); {code} Currently we use a HashMap {{Map<Long, ReplicaInfo>}} to store the replicas in memory. The key is block id of the block replica which is already included in {{ReplicaInfo}}, so this memory can be saved. Also HashMap Entry has a object overhead. We can implement a lightweight Set which is similar to {{LightWeightGSet}}, but not a fixed size ({{LightWeightGSet}} uses fix size for the entries array, usually it's a big value, an example is {{BlocksMap}}, this can avoid full gc since no need to resize), also we should be able to get Element through key. Following is comparison of memory footprint If we implement a lightweight set as described: We can save: {noformat} SIZE (bytes) ITEM 20 The Key: Long (12 bytes object overhead + 8 bytes long) 12 HashMap Entry object overhead 4 reference to the key in Entry 4 reference to the value in Entry 4 hash in Entry {noformat} Total: -44 bytes We need to add: {noformat} SIZE (bytes) ITEM 4 a reference to next element in ReplicaInfo {noformat} Total: +4 bytes So totally we can save 40bytes for each block replica And currently one finalized replica needs around 46 bytes (notice: we ignore memory alignment here). We can save 1 - (4 + 46) / (44 + 46) = *45%* memory for each block replica in DataNode. -- This message was sent by Atlassian JIRA (v6.3.4#6332)