Wei-Chiu Chuang created HDFS-14492:
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Summary: Snapshot memory leak
Key: HDFS-14492
URL: https://issues.apache.org/jira/browse/HDFS-14492
Project: Hadoop HDFS
Issue Type: Bug
Components: snapshots
Affects Versions: 2.6.0
Environment: CDH5.14.4
Reporter: Wei-Chiu Chuang
We recently examined the NameNode heap dump of a big, heavy snapshot user,
trying to trim some fat, and surely enough we found memory leak in it: when
snapshots are removed, the corresponding data structures are not removed.
This cluster has 586 million file system objects (286 million files, 287
million blocks, 13 million directories), using around 132gb of heap.
While only 44.5 million files have snapshotted copies,
(INodeFileAttributes$SnapshotCopy), most inodes (nearly 212 million) have
FileWithSnapshotFeature and FileDiffList. Those inodes had snapshotted copies
at some point in the past, but after snapshots are removed, those data
structured are still kept in the heap.
INode$Feature = 32.5 byte on average, FileWithSnapshotFeature = 32 bytes,
FileDiffList = 24 bytes. It may not sound a lot, but they add up quickly in
large clusters like this. In this cluster, a whopping 13.8gb of memory could
have been saved: ((32.5 + 32 + 24) bytes * (211997769 - 44572380) =~ 13.8gb)
if not for this bug. That is more than 10% of savings in heap size.
Heap histogram for reference:
{noformat}
num #instances #bytes class name
----------------------------------------------
1: 286418254 27496152384 org.apache.hadoop.hdfs.server.namenode.INodeFile
2: 287322227 18388622528
org.apache.hadoop.hdfs.server.blockmanagement.BlockInfo
3: 227899550 17144816120 [B
4: 287324031 13769408616
[Lorg.apache.hadoop.hdfs.server.blockmanagement.DatanodeStorageInfo;
5: 71352116 12353841568 [Ljava.lang.Object;
6: 286322650 9170335840
[Lorg.apache.hadoop.hdfs.server.blockmanagement.BlockInfo;
7: 235632329 7658462416 [Lorg.apache.hadoop.hdfs.server.namenode.INode$Feature;
8: 4 7046430816 [Lorg.apache.hadoop.util.LightWeightGSet$LinkedElement;
9: 211997769 6783928608
org.apache.hadoop.hdfs.server.namenode.snapshot.FileWithSnapshotFeature
10: 211997769 5087946456
org.apache.hadoop.hdfs.server.namenode.snapshot.FileDiffList
11: 76586261 3780468856 [I
12: 44572380 3209211360
org.apache.hadoop.hdfs.server.namenode.INodeFileAttributes$SnapshotCopy
13: 58634517 2345380680 java.util.ArrayList
14: 44572380 2139474240
org.apache.hadoop.hdfs.server.namenode.snapshot.FileDiff
15: 76582416 1837977984 org.apache.hadoop.hdfs.server.namenode.AclFeature
16: 12907668 1135874784
org.apache.hadoop.hdfs.server.namenode.INodeDirectory{noformat}
[~szetszwo] [~arpaga] [~smeng] [~shashikant] any thoughts?
I am thinking that inside
INodeFile#destroyAndCollectBlocks(), in addition to cleaning up file diffs, it
should also remove FileWithSnapshotFeature. I am not familiar with the snapshot
implementation, so any guidance is greatly appreciated.
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