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https://issues.apache.org/jira/browse/HDFS-9053?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14942004#comment-14942004
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Yi Liu commented on HDFS-9053:
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Thanks [~szetszwo], good comment, I ever considered it carefully too. I want to
convince you to allow me only use B-Tree here:
# Use the case you said, the #children is small and < 4K. *1)* If children is <
2K, then B-Tree only contains a root. As we counted before, the increased
overhead is only 44 bytes which is really very small for a directory, a
continuous block is 80 bytes memory (detail below), so we only increase about
1/2 continuous block for a directory in NN. *2)* If the children is > 2K and <
4K, here we use 4K as example, the B-Tree at most contains 3 branches: 1 root
node, 3 leaf nodes. One leaf node increase about (40 bytes + 16 bytes elements
array overhead) = 56 bytes, and 1 root node is (40 bytes + 16 bytes elements
array overhead + 16 bytes children overhead + 3 children * 8) = 96 bytes, the
b-tree itself is 40 bytes, and we need to subtract the ArrayList (40 bytes + 16
bytes elements array overhead) = 56 bytes, so we at most increase 56 * 3 + 96 +
40 - 56 = 248 bytes overhead, but ArrayList of 4K references to INode needs
more than 4K * 8 = 32K memory, then we can get that the increased memory is
only *0.75%*
{noformat}
org.apache.hadoop.hdfs.server.blockmanagement.BlockInfoContiguous object
internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 16 (object header) N/A
16 8 long Block.blockId N/A
24 8 long Block.numBytes N/A
32 8 long Block.generationStamp N/A
40 8 long BlockInfo.bcId N/A
48 2 short BlockInfo.replication N/A
50 6 (alignment/padding gap) N/A
56 8 LinkedElement BlockInfo.nextLinkedElement N/A
64 8 Object[] BlockInfo.triplets N/A
72 8 BlockUnderConstructionFeature BlockInfo.uc N/A
Instance size: 80 bytes (estimated, the sample instance is not available)
{noformat}
# One advantage of B-Tree compared to ArrayList for small size children is:
B-Tree can shrink. If the children of directory decreases from 4K to 2K, there
are 2K * 8 = 16K memory wasteful if suing ArrayList.
# On the other hand, if we do the switch between ArrayList and B-Tree, we may
need write a class to wrap the two data structures, then it still needs
16bytes object overhead + 8 bytes additional reference = 24 bytes.
How do you say? Thanks, Nicholas.
> 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,
> HDFS-9053.004.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, the time complexity is
> O\(n), (the search is O(log n), but insertion/deleting causes re-allocations
> and copies of arrays), for large directory, the operations are expensive. If
> the children grow to 1M size, the ArrayList will resize to > 1M capacity, so
> need > 1M * 8bytes = 8M (the reference size is 8 for 64-bits system/JVM)
> 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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