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From "Brahma Reddy Battula (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8859) Improve DataNode ReplicaMap memory footprint to save about 45%
Date Fri, 25 Sep 2015 02:20:04 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8859?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14907423#comment-14907423
] 

Brahma Reddy Battula commented on HDFS-8859:
--------------------------------------------

Hi [~hitliuyi], Thank you for working on this. Nice work here..
   I have another nit :  {{LightWeightResizableGset}} need not override iterator as super
class implementation is sufficient

> 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
>
>
> 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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