[jira] [Created] (HDFS-8859) Improve DataNode (ReplicaMap) memory footprint to save about 45%

["Yi Liu (JIRA)" <ji...@apache.org>]

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.
    



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