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https://issues.apache.org/jira/browse/HDFS-8859?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14934768#comment-14934768
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Uma Maheswara Rao G commented on HDFS-8859:
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Thanks Yi, +1 on the latest patch
> 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,
> HDFS-8859.006.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.
>
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