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From "Yi Liu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9053) Support large directories efficiently using B-Tree
Date Wed, 30 Sep 2015 00:56:04 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9053?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14936183#comment-14936183

Yi Liu commented on HDFS-9053:

Thanks [~szetszwo] for the comments.

here are quite a few errors in description and the analysis. Let's hold on the patch for the
moment and fix all these errors first.
Sure, let's fix the errors in the description and the analysis first, thanks for pointing
out this.

> So totally 12+4+4+4+4+4+4 = 32 bytes on 32-bits system/JVM, and 12+4+8+4+8+4 = 40 bytes
on 64-bits system/JVM. (I have not counted object alignment)
It seems that there is at least one more reference not yet counted since Node is non-static.
The reference to Node is already counted, use 64-bits system/JVM as example, the first '8'
is the reference to the B-Tree root.  The second '8' is the null reference for children in
BTree#Node, of course we are talking about the directory which is not large and only contains
one B-Tree node.

Nicholas, I am going to fix the two places in the JIRA description and analysis in JIRA comments,
which we find so far
# fix the time complexity of insert/delete for ArrayList
# fix the description about memory usage for B-Tree to add the system/JVM information and
add more detailed information.

Any other errors do you find besides these? please tell me and appreciate if you can also
help to edit them directly.

> 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
> 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/search, the time complexity is O(log n), but insertion/deleting causes
re-allocations and copies of big arrays, so the operations are costly.  For example, if the
children grow to 1M size, the ArrayList will resize to > 1M capacity, so need > 1M *
4bytes = 4M 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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