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https://issues.apache.org/jira/browse/HDFS-9053?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14740842#comment-14740842
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Yi Liu edited comment on HDFS-9053 at 9/15/15 12:35 AM:
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To integrate B-Tree with INodeDirectory, it's natural besides directory exposes
{{getChildrenList}} and returns {{ReadOnlyList}}, B-Tree will be updated to
implement Collection (now it implements Iterable), it's not a list. Actually
most other places want to iterate all elements, and some places want to iterate
starting from certain child. So my plan is adding {{ReadOnlyCollection}} and
{{ReadOnlyList}} extends it, and getChildren returns {{ReadOnlyCollection}}.
The {{ReadOnlyCollection}} has an interface to allow creating an Iterator
starting from certain child. For snapshot, it makes sense to keep current
behavior, still use list to keep the CREATED/DELETED diff.
was (Author: hitliuyi):
To integrate B-Tree with INodeDirectory, it's natural besides directory exposes
{{getChildrenList}} and returns {{ReadOnlyList}}, B-Tree implements Iterable,
it's not a list. Actually most other places want to iterate all elements, and
some places want to iterate starting from certain child. So my plan is add an
new Iterable interface which extends {{java.lang.Iterable}} and allow creating
an iterator starting from certain child. Then in directory, getChildren returns
{{ReadOnlyIterator}}, and {{ReadOnlyList}} implements it the ReadOnlyIterator.
For snapshot, it makes sense to keep current behavior, still use the
{{ReadOnlyList}}.
> 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
>
>
> 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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