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https://issues.apache.org/jira/browse/HDFS-12042?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16065205#comment-16065205
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Manoj Govindassamy commented on HDFS-12042:
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[[email protected]],
I believe lazy initialization has much bigger impact than replacing it with
a simple object or the ArrayList. There is a very high probability for a
file/dir not being part of Snapshot tree. But once it is part of the snapshot
tree, the likelihood of seeing more than one snapshot is quite high. So IMHO,
the second optimization of simple object or ArrayList might not be so useful.
> Reduce memory used by snapshot diff data structures
> ---------------------------------------------------
>
> Key: HDFS-12042
> URL: https://issues.apache.org/jira/browse/HDFS-12042
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Misha Dmitriev
> Assignee: Misha Dmitriev
> Attachments: HDFS-12042.01.patch, HDFS-12042.02.patch
>
>
> When snapshot diff operation is performed in a NameNode that manages several
> million HDFS files/directories, NN needs a lot of memory. Some of that memory
> is wasted due to suboptimal data structures, such as empty or under-populated
> ArrayLists, etc. Analyzing one heap dump with jxray (www.jxray.com), we
> observed the following problems with data structures:
> {code}
> 9. BAD COLLECTIONS
> Total collections: 99,707,902 Bad collections: 88,799,760 Overhead:
> 9,063,898K (18.2%)
> Top bad collections:
> Ovhd Problem Num objs Type
> -------------------------------------------------
> 3,056,014K (6.1%) small 29435572 j.u.ArrayList
> 2,641,373K (5.3%) 1-elem 21837906 j.u.ArrayList
> 864,215K (1.7%) 1-elem 5291813 j.u.TreeSet
> 808,456K (1.6%) 1-elem 3045847 j.u.HashMap
> 602,470K (1.2%) empty 18549109 j.u.ArrayList
> 441,563K (0.9%) empty 4356975 j.u.TreeSet
> 373,088K (0.7%) empty 5297007 j.u.HashMap
> 270,324K (0.5%) small 931394 j.u.HashMap
> {code}
> The data structures created by HDFS code that suffer from the above problems
> are, in particular:
> {code}
> 4,228,182K (8.5%): j.u.ArrayList: 19412263 of small 2,111,087K (4.2%),
> 12932408 of 1-elem 1,717,585K (3.4%), 12784310 of empty 399,509K (0.8%)
> <-- org.apache.hadoop.hdfs.server.namenode.snapshot.FileDiffList.diffs
> <--
> org.apache.hadoop.hdfs.server.namenode.snapshot.FileWithSnapshotFeature.diffs
> <-- org.apache.hadoop.hdfs.server.namenode.INode$Feature[] <--
> org.apache.hadoop.hdfs.server.namenode.INodeFile.features <--
> org.apache.hadoop.hdfs.server.blockmanagement.BlockInfo.bc <--
> org.apache.hadoop.util.LightWeightGSet$LinkedElement[] <--
> org.apache.hadoop.hdfs.server.blockmanagement.BlocksMap$1.entries <--
> org.apache.hadoop.hdfs.server.blockmanagement.BlocksMap.blocks <--
> org.apache.hadoop.hdfs.server.blockmanagement.BlocksMap$1.entries <--
> org.apache.hadoop.hdfs.server.blockmanagement.BlocksMap.blocks <--
> org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.blocksMap <--
> org.apache.hadoop.hdfs.server.blockmanagement.BlockManager$BlockReportProcessingThread.this$0
> <-- j.l.Thread[] <-- j.l.ThreadGroup.threads <-- j.l.Thread.group <-- Java
> Static: org.apache.hadoop.fs.FileSystem$Statistics.STATS_DATA_CLEANER
> {code}
> and
> {code}
> 575,557K (1.2%): j.u.ArrayList: 4363271 of 1-elem 409,056K (0.8%), 2439001
> of small 166,482K (0.3%)
> <-- org.apache.hadoop.hdfs.server.namenode.INodeDirectory.children <--
> org.apache.hadoop.util.LightWeightGSet$LinkedElement[] <--
> org.apache.hadoop.util.LightWeightGSet.entries <--
> org.apache.hadoop.hdfs.server.namenode.INodeMap.map <--
> org.apache.hadoop.hdfs.server.namenode.FSDirectory.inodeMap <--
> org.apache.hadoop.hdfs.server.namenode.FSNamesystem.dir <--
> org.apache.hadoop.hdfs.server.namenode.FSNamesystem$NameNodeResourceMonitor.this$0
> <-- org.apache.hadoop.util.Daemon.target <--
> org.apache.hadoop.hdfs.server.namenode.FSDirectory.inodeMap <--
> org.apache.hadoop.hdfs.server.namenode.FSNamesystem.dir <--
> org.apache.hadoop.hdfs.server.namenode.FSNamesystem$NameNodeResourceMonitor.this$0
> <-- org.apache.hadoop.util.Daemon.target <-- j.l.Thread[] <--
> j.l.ThreadGroup.threads <-- j.l.Thread.group <-- Java Static:
> org.apache.hadoop.fs.FileSystem$Statistics.STATS_DATA_CLEANER
> {code}
> There are several different reference chains that all lead to
> FileDiffList.diffs or INodeDirectory.children. The total percentage of memory
> wasted by these data structures in the analyzed dump is about 12%. By
> creating these lists lazily and/or with capacity that better matches their
> actual size, we should be able to reclaim a significant part of these 12%.
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