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Yi Liu commented on HDFS-8859: ------------------------------ Thanks Uma. There is an unused import, I will remove it in the new version of patch. {quote} ./hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/LightWeightGSet.java:69:29: Variable 'entries' must be private and have accessor methods. ./hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/LightWeightGSet.java:71:17: Variable 'hash_mask' must be private and have accessor methods. ./hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/LightWeightGSet.java:73:17: Variable 'size' must be private and have accessor methods. ./hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/util/LightWeightGSet.java:77:17: Variable 'modification' must be private and have accessor methods. {quote} Making the variables of super class 'protected' and modify them in sub classes is a natural behavior, I don't know why checkstype reports we should use private and access through methods. We always access the protected variables in the super class directly in other hadoop code. So I will leave these checkstyle items. > 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 > Attachments: HDFS-8859.001.patch, HDFS-8859.002.patch, > HDFS-8859.003.patch, HDFS-8859.004.patch, HDFS-8859.005.patch > > > 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)