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From "Tsz Wo Nicholas Sze (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9053) Support large directories efficiently using B-Tree
Date Fri, 16 Oct 2015 01:01:05 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9053?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14959972#comment-14959972
] 

Tsz Wo Nicholas Sze commented on HDFS-9053:
-------------------------------------------

> For small elements size (assume # < max degree which is 2047), ... Do I miss something?

According to [your comment|https://issues.apache.org/jira/browse/HDFS-9053?focusedCommentId=14950498&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14950498]
(also copied below), you were saying that B-Tree only increased 8 bytes when #children <
4K, i.e. when 2047 < #children < 4K.  Is it still true?  If not, how much memory is
needed when 2047 < #children < 4K?
{quote}
I find a good approach to improve B-Tree memory overhead to make it only increase 8 bytes
memory usage comparing with using ArrayList for small elements size.
So we don't need to use ArrayList when #children is small (< 4K), and we can always use
the BTree.
{quote}

> 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,
HDFS-9053.006.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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