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https://issues.apache.org/jira/browse/HDFS-9053?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14901761#comment-14901761
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Jing Zhao commented on HDFS-9053:
---------------------------------
Thanks for the great work, Yi! So far I just reviewed the B-Tree implementation
part and it looks good to me. Just some minor comments:
# "static" can be removed
{code}
public static interface Element<K> extends Comparable<K> {
K getKey();
}
{code}
# The parameter is never used.
{code}
Node(boolean allocateMaxElements) {
elements = new Object[maxElements()];
}
{code}
# It may be helpful to add some more Preconditions/assert check to verify the
parameter and internal state. For example, some verification about the index i
in the following code.
{code}
SplitResult split(int i) {
E e = (E)elements[i];
Node next = new Node(true);
....
{code}
# Optional: in insertElement maybe we can copy elements only once if we need to
expand the array.
# Rename {{put}} to {{addOrReplace}} to make its semantic more clear?
# Need to update the javadoc of {{removeElement}} and {{removeChild}}.
# {{SplitResult#element}} and {{SplitResult#node}} can be declared as final.
> 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
>
>
> 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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