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https://issues.apache.org/jira/browse/HADOOP-2095?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12595386#action_12595386
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eric baldeschwieler commented on HADOOP-2095:
---------------------------------------------
To clarify some of the thinking above... The short term goal is not to find
the optimal solution. It is to get something done that is clean,
understandable and works acceptably well in all cases. We can refine from
there.
to expand on the above suggestions:
I suggest that for objects larger than 25% of RAM, we always just send them
directly to disk. A simple rule that let's us reason more easily about other
case. I don't think the 10% number above can be replaced with thiis.
We need to understand how we pause the copies without doing lots of polling.
Again, I suggest keeping it simple for now. What about simply setting a global
flag the first time a thread starts to read an input that is < 25% of buffer
RAM (not piped directly to disk) and doesn't fit in the remaining space. Other
readers will then pause until this semaphore is cleared. It is ok if races
happen where a few threads try at the same time. If copies fail, we will need
to clear this semaphore too.
We want to be sure not to wait until RAM is totally full before starting the
merge, because this might allow a single slow copy to brownout the system. I
suggest a simple rule, such as wait until the semaphore discussed above is set
and copies filling at least 50% of RAM have completed. Then merge.
Once all of the above is done, we can file new jiras to improve things. Ideas
include:
- Freeing storage as we merge, so fetches can be interleaved
- decompressing small segments as we read so we can increase the number of
compressed objects merged
- ...
> Reducer failed due to Out ofMemory
> ----------------------------------
>
> Key: HADOOP-2095
> URL: https://issues.apache.org/jira/browse/HADOOP-2095
> Project: Hadoop Core
> Issue Type: Bug
> Components: mapred
> Affects Versions: 0.15.0
> Reporter: Runping Qi
> Assignee: Arun C Murthy
> Attachments: HADOOP-2095_CompressedBytesWithCodecPool.patch,
> HADOOP-2095_debug.patch
>
>
> One of the reducers of my job failed with the following exceptions.
> The failure caused the whole job fail eventually.
> Java heapsize was 768MB and sort.io.mb was 140.
> 2007-10-23 19:24:06,100 WARN org.apache.hadoop.mapred.ReduceTask:
> task_200710231912_0001_r_000020_2 Intermediate Merge of the inmemory files
> threw an exception: java.lang.OutOfMemoryError: Java heap space
> at
> org.apache.hadoop.io.compress.DecompressorStream.(DecompressorStream.java:43)
> at
> org.apache.hadoop.io.compress.DefaultCodec.createInputStream(DefaultCodec.java:71)
> at org.apache.hadoop.io.SequenceFile$Reader.init(SequenceFile.java:1345)
> at org.apache.hadoop.io.SequenceFile$Reader.(SequenceFile.java:1231)
> at org.apache.hadoop.io.SequenceFile$Reader.(SequenceFile.java:1154)
> at
> org.apache.hadoop.io.SequenceFile$Sorter$SegmentDescriptor.nextRawKey(SequenceFile.java:2726)
> at
> org.apache.hadoop.io.SequenceFile$Sorter$MergeQueue.merge(SequenceFile.java:2543)
> at
> org.apache.hadoop.io.SequenceFile$Sorter.merge(SequenceFile.java:2297)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$InMemFSMergeThread.run(ReduceTask.java:1311)
> 2007-10-23 19:24:06,102 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200710231912_0001_r_000020_2 done copying
> task_200710231912_0001_m_001428_0 output .
> 2007-10-23 19:24:06,185 INFO org.apache.hadoop.fs.FileSystem: Initialized
> InMemoryFileSystem:
> ramfs://mapoutput31952838/task_200710231912_0001_r_000020_2/map_1423.out-0 of
> size (in bytes): 209715200
> 2007-10-23 19:24:06,193 ERROR org.apache.hadoop.mapred.ReduceTask: Map output
> copy failure: java.lang.NullPointerException
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$FileAttributes.access$300(InMemoryFileSystem.java:366)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$InMemoryFileStatus.(InMemoryFileSystem.java:378)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem.getFileStatus(InMemoryFileSystem.java:283)
> at
> org.apache.hadoop.fs.FilterFileSystem.getFileStatus(FilterFileSystem.java:251)
> at org.apache.hadoop.fs.FileSystem.getLength(FileSystem.java:449)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(ReduceTask.java:738)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTask.java:665)
> 2007-10-23 19:24:06,193 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200710231912_0001_r_000020_2 Copying task_200710231912_0001_m_001215_0
> output from xxx
> 2007-10-23 19:24:06,188 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200710231912_0001_r_000020_2 Copying task_200710231912_0001_m_001211_0
> output from xxx
> 2007-10-23 19:24:06,185 ERROR org.apache.hadoop.mapred.ReduceTask: Map output
> copy failure: java.lang.NullPointerException
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$InMemoryOutputStream.close(InMemoryFileSystem.java:161)
> at
> org.apache.hadoop.fs.FSDataOutputStream$PositionCache.close(FSDataOutputStream.java:49)
> at
> org.apache.hadoop.fs.FSDataOutputStream.close(FSDataOutputStream.java:64)
> at
> org.apache.hadoop.fs.ChecksumFileSystem$ChecksumFSOutputSummer.close(ChecksumFileSystem.java:312)
> at
> org.apache.hadoop.fs.FSDataOutputStream$PositionCache.close(FSDataOutputStream.java:49)
> at
> org.apache.hadoop.fs.FSDataOutputStream.close(FSDataOutputStream.java:64)
> at
> org.apache.hadoop.mapred.MapOutputLocation.getFile(MapOutputLocation.java:253)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(ReduceTask.java:713)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTask.java:665)
> 2007-10-23 19:24:06,199 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200710231912_0001_r_000020_2 Copying task_200710231912_0001_m_001247_0
> output from .
> 2007-10-23 19:24:06,200 ERROR org.apache.hadoop.mapred.ReduceTask: Map output
> copy failure: java.lang.NullPointerException
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$FileAttributes.access$300(InMemoryFileSystem.java:366)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$InMemoryFileStatus.(InMemoryFileSystem.java:378)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem.getFileStatus(InMemoryFileSystem.java:283)
> at
> org.apache.hadoop.fs.FilterFileSystem.getFileStatus(FilterFileSystem.java:251)
> at org.apache.hadoop.fs.FileSystem.getLength(FileSystem.java:449)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(ReduceTask.java:738)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTask.java:665)
> 2007-10-23 19:24:06,204 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200710231912_0001_r_000020_2 Copying task_200710231912_0001_m_001422_0
> output from .
> 2007-10-23 19:24:06,207 ERROR org.apache.hadoop.mapred.ReduceTask: Map output
> copy failure: java.lang.NullPointerException
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$FileAttributes.access$300(InMemoryFileSystem.java:366)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$InMemoryFileStatus.(InMemoryFileSystem.java:378)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem.getFileStatus(InMemoryFileSystem.java:283)
> at
> org.apache.hadoop.fs.FilterFileSystem.getFileStatus(FilterFileSystem.java:251)
> at org.apache.hadoop.fs.FileSystem.getLength(FileSystem.java:449)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(ReduceTask.java:738)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTask.java:665)
> 2007-10-23 19:24:06,209 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200710231912_0001_r_000020_2 Copying task_200710231912_0001_m_001278_0
> output from .
> 2007-10-23 19:24:06,198 WARN org.apache.hadoop.mapred.TaskTracker: Error
> running child
> java.io.IOException: task_200710231912_0001_r_000020_2The reduce copier failed
> at org.apache.hadoop.mapred.ReduceTask.run(ReduceTask.java:253)
> at
> org.apache.hadoop.mapred.TaskTracker$Child.main(TaskTracker.java:1760)
> 2007-10-23 19:24:06,198 ERROR org.apache.hadoop.mapred.ReduceTask: Map output
> copy failure: java.lang.NullPointerException
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$FileAttributes.access$300(InMemoryFileSystem.java:366)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$InMemoryFileStatus.(InMemoryFileSystem.java:378)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem.getFileStatus(InMemoryFileSystem.java:283)
> at
> org.apache.hadoop.fs.FilterFileSystem.getFileStatus(FilterFileSystem.java:251)
> at org.apache.hadoop.fs.FileSystem.getLength(FileSystem.java:449)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(ReduceTask.java:738)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTask.java:665)
> 2007-10-23 19:24:06,231 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200710231912_0001_r_000020_2 Copying task_200710231912_0001_m_001531_0
> output from .
> 2007-10-23 19:24:06,197 ERROR org.apache.hadoop.mapred.ReduceTask: Map output
> copy failure: java.lang.NullPointerException
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$FileAttributes.access$300(InMemoryFileSystem.java:366)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$InMemoryFileStatus.(InMemoryFileSystem.java:378)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem.getFileStatus(InMemoryFileSystem.java:283)
> at
> org.apache.hadoop.fs.FilterFileSystem.getFileStatus(FilterFileSystem.java:251)
> at org.apache.hadoop.fs.FileSystem.getLength(FileSystem.java:449)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(ReduceTask.java:738)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTask.java:665)
> 2007-10-23 19:24:06,237 INFO org.apache.hadoop.mapred.ReduceTask:
> task_200710231912_0001_r_000020_2 Copying task_200710231912_0001_m_001227_0
> output from .
> 2007-10-23 19:24:06,196 ERROR org.apache.hadoop.mapred.ReduceTask: Map output
> copy failure: java.lang.NullPointerException
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$FileAttributes.access$300(InMemoryFileSystem.java:366)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem$InMemoryFileStatus.(InMemoryFileSystem.java:378)
> at
> org.apache.hadoop.fs.InMemoryFileSystem$RawInMemoryFileSystem.getFileStatus(InMemoryFileSystem.java:283)
> at
> org.apache.hadoop.fs.FilterFileSystem.getFileStatus(FilterFileSystem.java:251)
> at org.apache.hadoop.fs.FileSystem.getLength(FileSystem.java:449)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(ReduceTask.java:738)
> at
> org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTask.java:665)
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