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From "Yi Liu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9053) Support large directories efficiently using B-Tree
Date Fri, 09 Oct 2015 08:48:26 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9053?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14950084#comment-14950084
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Yi Liu commented on HDFS-9053:
------------------------------

The increased memory usage is only 8 bytes in {{SortedCollection}} comparing with {{ArrayList}}
if the elements size is small (<= 4K).
{noformat}
java.util.ArrayList object internals:
 OFFSET  SIZE     TYPE DESCRIPTION                    VALUE
      0    16          (object header)                N/A
     16     4      int AbstractList.modCount          N/A
     20     4          (alignment/padding gap)        N/A
     24     4      int ArrayList.size                 N/A
     28     4          (alignment/padding gap)        N/A
     32     8 Object[] ArrayList.elementData          N/A
Instance size: 40 bytes (estimated, the sample instance is not available)
{noformat}

{noformat}
org.apache.hadoop.hdfs.util.SortedCollection object internals:
 OFFSET  SIZE     TYPE DESCRIPTION                    VALUE
      0    16          (object header)                N/A
     16     4      int SortedCollection.initCapacity  N/A
     20     4      int SortedCollection.size          N/A
     24     4      int SortedCollection.modCount      N/A
     28     4      int SortedCollection.degree        N/A
     32     8 Object[] SortedCollection.elements      N/A
     40     8    BTree SortedCollection.btree         N/A
Instance size: 48 bytes (estimated, the sample instance is not available)
{noformat}

Hi [~jingzhao], [~szetszwo], could you review the new patch? Thanks.

> Support large directories efficiently using B-Tree
> --------------------------------------------------
>
>                 Key: HDFS-9053
>                 URL: https://issues.apache.org/jira/browse/HDFS-9053
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: namenode
>            Reporter: Yi Liu
>            Assignee: Yi Liu
>            Priority: Critical
>         Attachments: HDFS-9053 (BTree with simple benchmark).patch, HDFS-9053 (BTree).patch,
HDFS-9053.001.patch, HDFS-9053.002.patch, HDFS-9053.003.patch, HDFS-9053.004.patch, HDFS-9053.005.patch
>
>
> This is a long standing issue, we were trying to improve this in the past.  Currently
we use an ArrayList for the children under a directory, and the children are ordered in the
list, for insert/delete, the time complexity is O\(n), (the search is O(log n), but insertion/deleting
causes re-allocations and copies of arrays), for large directory, the operations are expensive.
 If the children grow to 1M size, the ArrayList will resize to > 1M capacity, so need >
1M * 8bytes = 8M (the reference size is 8 for 64-bits system/JVM) continuous heap memory,
it easily causes full GC in HDFS cluster where namenode heap memory is already highly used.
 I recap the 3 main issues:
> # Insertion/deletion operations in large directories are expensive because re-allocations
and copies of big arrays.
> # Dynamically allocate several MB continuous heap memory which will be long-lived can
easily cause full GC problem.
> # Even most children are removed later, but the directory INode still occupies same size
heap memory, since the ArrayList will never shrink.
> This JIRA is similar to HDFS-7174 created by [~kihwal], but use B-Tree to solve the problem
suggested by [~shv]. 
> So the target of this JIRA is to implement a low memory footprint B-Tree and use it to
replace ArrayList. 
> If the elements size is not large (less than the maximum degree of B-Tree node), the
B-Tree only has one root node which contains an array for the elements. And if the size grows
large enough, it will split automatically, and if elements are removed, then B-Tree nodes
can merge automatically (see more: https://en.wikipedia.org/wiki/B-tree).  It will solve the
above 3 issues.



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