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Colin Patrick McCabe updated HDFS-8562: --------------------------------------- Attachment: (was: HDFS-8562.002.patch) > HDFS Performance is impacted by FileInputStream Finalizer > --------------------------------------------------------- > > Key: HDFS-8562 > URL: https://issues.apache.org/jira/browse/HDFS-8562 > Project: Hadoop HDFS > Issue Type: Improvement > Components: datanode, performance > Affects Versions: 2.5.0 > Environment: Impact any application that uses HDFS > Reporter: Yanping Wang > Assignee: Wei Zhou > Attachments: HDFS-8562.002b.patch, HDFS-8562.01.patch > > > While running HBase using HDFS as datanodes, we noticed excessive high GC > pause spikes. For example with jdk8 update 40 and G1 collector, we saw > datanode GC pauses spiked toward 160 milliseconds while they should be around > 20 milliseconds. > We tracked down to GC logs and found those long GC pauses were devoted to > process high number of final references. > For example, this Young GC: > 2715.501: [GC pause (G1 Evacuation Pause) (young) 0.1529017 secs] > 2715.572: [SoftReference, 0 refs, 0.0001034 secs] > 2715.572: [WeakReference, 0 refs, 0.0000123 secs] > 2715.572: [FinalReference, 8292 refs, 0.0748194 secs] > 2715.647: [PhantomReference, 0 refs, 160 refs, 0.0001333 secs] > 2715.647: [JNI Weak Reference, 0.0000140 secs] > [Ref Proc: 122.3 ms] > [Eden: 910.0M(910.0M)->0.0B(911.0M) Survivors: 11.0M->10.0M Heap: > 951.1M(1536.0M)->40.2M(1536.0M)] > [Times: user=0.47 sys=0.01, real=0.15 secs] > This young GC took 152.9 milliseconds STW pause, while spent 122.3 > milliseconds in Ref Proc, which processed 8292 FinalReference in 74.8 > milliseconds plus some overhead. > We used JFR and JMAP with Memory Analyzer to track down and found those > FinalReference were all from FileInputStream. We checked HDFS code and saw > the use of the FileInputStream in datanode: > https://apache.googlesource.com/hadoop-common/+/refs/heads/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/MappableBlock.java > {code} > 1. public static MappableBlock load(long length, > 2. FileInputStream blockIn, FileInputStream metaIn, > 3. String blockFileName) throws IOException { > 4. MappableBlock mappableBlock = null; > 5. MappedByteBuffer mmap = null; > 6. FileChannel blockChannel = null; > 7. try { > 8. blockChannel = blockIn.getChannel(); > 9. if (blockChannel == null) { > 10. throw new IOException("Block InputStream has no FileChannel."); > 11. } > 12. mmap = blockChannel.map(MapMode.READ_ONLY, 0, length); > 13. NativeIO.POSIX.getCacheManipulator().mlock(blockFileName, mmap, length); > 14. verifyChecksum(length, metaIn, blockChannel, blockFileName); > 15. mappableBlock = new MappableBlock(mmap, length); > 16. } finally { > 17. IOUtils.closeQuietly(blockChannel); > 18. if (mappableBlock == null) { > 19. if (mmap != null) { > 20. NativeIO.POSIX.munmap(mmap); // unmapping also unlocks > 21. } > 22. } > 23. } > 24. return mappableBlock; > 25. } > {code} > We looked up > https://docs.oracle.com/javase/7/docs/api/java/io/FileInputStream.html and > http://hg.openjdk.java.net/jdk7/jdk7/jdk/file/23bdcede4e39/src/share/classes/java/io/FileInputStream.java > and noticed FileInputStream relies on the Finalizer to release its resource. > When a class that has a finalizer created, an entry for that class instance > is put on a queue in the JVM so the JVM knows it has a finalizer that needs > to be executed. > The current issue is: even with programmers do call close() after using > FileInputStream, its finalize() method will still be called. In other words, > still get the side effect of the FinalReference being registered at > FileInputStream allocation time, and also reference processing to reclaim the > FinalReference during GC (any GC solution has to deal with this). > We can imagine When running industry deployment HDFS, millions of files could > be opened and closed which resulted in a very large number of finalizers > being registered and subsequently being executed. That could cause very long > GC pause times. > We tried to use Files.newInputStream() to replace FileInputStream, but it was > clear we could not replace FileInputStream in > hdfs/server/datanode/fsdataset/impl/MappableBlock.java > We notified Oracle JVM team of this performance issue that impacting all Big > Data applications using HDFS. We recommended the proper fix in Java SE > FileInputStream. Because (1) it is really nothing wrong to use > FileInputStream in above datanode code, (2) as the object with a finalizer is > registered with finalizer list within the JVM at object allocation time, if > someone makes an explicit call to close or free the resources that are to be > done in the finalizer, then the finalizer should be pulled off the internal > JVM’s finalizer list. That will release the JVM from having to treat the > object as special because it has a finalizer, i.e. no need for GC to execute > the finalizer as part of Reference Processing. > As the java fix involves both JVM code and Java SE code, it might take time > for the full solution to be available in future JDK releases. We would like > to file his JIRA to notify Big Data, HDFS community to aware this issue while > using HDFS and while writing code using FileInputStream > One alternative is to use Files.newInputStream() to substitute > FileInputStream if it is possible. File.newInputStream() will give an > InputStream and do so in a manner that does not include a finalizer. > We welcome HDFS community to discuss this issue and see if there are > additional ideas to solve this problem. -- This message was sent by Atlassian JIRA (v6.3.4#6332)