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https://issues.apache.org/jira/browse/HDFS-8562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15058718#comment-15058718
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Colin Patrick McCabe commented on HDFS-8562:
--------------------------------------------
That's frustrating. Unfortunately, I don't think there is any file-related
class in standard Java which can be created via a {{FileDescriptor}} without
using private APIs-- except {{FileInputStream}}. {{FileChannel#open}} only
takes paths. {{RandomAccessFile}}'s constructors only take paths as well. One
approach would be to try to use the private APIs via reflection, and then fall
back to creating a {{FileInputStream}} if that's not possible. We can get a
{{FileChannel}} from the {{FileInputStream}}. In order to do that, we'd have
to hold around a reference to the {{FileInputStream}} somewhere, to prevent its
finalizer from kicking in and closing the {{FileDescriptor}}.
> 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
> 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.
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