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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
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Yi Liu commented on HDFS-9053:
------------------------------

Thanks [~szetszwo] for the comments.

{quote}
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.
{quote}
Sure, let's fix the errors in the description and the analysis first, thanks for pointing
out this.

{quote}
> 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.
{quote}
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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