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https://issues.apache.org/jira/browse/HDFS-9053?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14934689#comment-14934689
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Yi Liu commented on HDFS-9053:
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[~jingzhao], you have given a good review, and it hits the two places I ever
considered carefully how to do it better. Thanks.
{quote}
In INodeDirectory#replaceChild, can we directly call addOrReplace instead of
calling get first?
{quote}
I think we can use {{addOrReplace}} directly. Since there should be an INode
with the name existing. To keep original behavior, I will remove the added one
if {{addOrReplace}} returns null.
{quote}
Do you think we can avoid the following code? Maybe we can add the EK type to
the ReadOnlyCollection/ReadOnlyList level?
{quote}
That's a good comment, I ever considered this carefully. I also thought adding
the EK type as one of generic type to the ReadOnlyCollection/ReadOnlyList
level, but I felt it looked not natural for a collection/list, and not all
implementations of ReadOnlyList need to implement iterating starting from
specified element, also I though it was OK since it's a private interface we
use in HDFS. I will leave this comment in next version of patch, if you feel
we'd better to do this, I will update it, I am OK with the both ways.
{quote}
DirectoryWithSnapshotFeature#getChildrenList#iterator(EK) forgot to increase
pos? Maybe also add a new test for this (e.g., set a small ls limit and list a
snapshot of a directory)?
{quote}
Great catch, let me update it, and add a new test in {{TestLargeDirectory}} to
cover.
{quote}
In getListing, instead of continuing the iteration, can we just call size() to
calculate the number of the remaining items?
{quote}
I ever tried to find a better way. {{size()}} will return the total number of
elements in B-Tree, but we don't know the current index, so seems not able to
calculate the number of the remaining items.
> 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
>
>
> 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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