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Moving to mapreduce-user@, bcc common-user@. Please use project specific lists.

Niranjan,

If you average as 0.5G output per-map, it's 5000 maps *0.5G -> 2.5TB over 12 reduces i.e. nearly 250G per reduce - compressed!

If you think you have 4:1 compression you are doing nearly a Terabyte per reducer... which is way too high!

I'd recommend you bump to somewhere along 1000 reduces to get to 2.5G (compressed) per reducer for your job. If your compression ratio is 2:1, try 500 reduces and so on.

If you are worried about other users, use the CapacityScheduler and submit your job to a queue with a small capacity and max-capacity to restrict your job to 10 or 20 concurrent reduces at a given point.

Arun
On Dec 7, 2011, at 10:51 AM, Niranjan Balasubramanian wrote:

All

I am encountering the following out-of-memory error during the reduce phase of a large job.

Map output copy failure : java.lang.OutOfMemoryError: Java heap space
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.shuffleInMemory(ReduceTask.java:1669)
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.getMapOutput(ReduceTask.java:1529)
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(ReduceTask.java:1378)
at org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTask.java:1310)
I tried increasing the memory available using mapped.child.java.opts but that only helps a little. The reduce task eventually fails again. Here are some relevant job configuration details:

1. The input to the mappers is about 2.5 TB (LZO compressed). The mappers filter out a small percentage of the input ( less than 1%).

2. I am currently using 12 reducers and I can't increase this count by much to ensure availability of reduce slots for other users.

3. mapred.child.java.opts --> -Xms512M -Xmx1536M -XX:+UseSerialGC

4. mapred.job.shuffle.input.buffer.percent --> 0.70

5. mapred.job.shuffle.merge.percent --> 0.66

6. mapred.inmem.merge.threshold --> 1000

7. I have nearly 5000 mappers which are supposed to produce LZO compressed outputs. The logs seem to indicate that the map outputs range between 0.3G to 0.8GB.

Does anything here seem amiss? I'd appreciate any input of what settings to try. I can try different reduced values for the input buffer percent and the merge percent. Given that the job runs for about 7-8 hours before crashing, I would like to make some informed choices if possible.

Thanks.
~ Niranjan.

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  • Koji Noguchi at Dec 10, 2011 at 8:13 am ⇧
    Hi Niranjan,

    If you're using jvm version of 1.6.018 or newer, then setting

    -XX:NewRatio=8

    might help.

    Some background.
    http://www.mail-archive.com/common-user@hadoop.apache.org/msg11286.html
    4. mapred.job.shuffle.input.buffer.percent --> 0.70
    5. mapred.job.shuffle.merge.percent --> 0.66
    These conf values may be too high after jvm's default NewRatio changed to 2.

    Koji

    On 12/9/11 10:29 AM, "Arun C Murthy" wrote:

    Moving to mapreduce-user@, bcc common-user@. Please use project specific
    lists.

    Niranjan,

    If you average as 0.5G output per-map, it's 5000 maps *0.5G -> 2.5TB over 12
    reduces i.e. nearly 250G per reduce - compressed!

    If you think you have 4:1 compression you are doing nearly a Terabyte per
    reducer... which is way too high!

    I'd recommend you bump to somewhere along 1000 reduces to get to 2.5G
    (compressed) per reducer for your job. If your compression ratio is 2:1, try
    500 reduces and so on.

    If you are worried about other users, use the CapacityScheduler and submit
    your job to a queue with a small capacity and max-capacity to restrict your
    job to 10 or 20 concurrent reduces at a given point.

    Arun
    On Dec 7, 2011, at 10:51 AM, Niranjan Balasubramanian wrote:

    All

    I am encountering the following out-of-memory error during the reduce phase
    of a large job.

    Map output copy failure : java.lang.OutOfMemoryError: Java heap space
    at
    org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.shuffleInMem
    ory(ReduceTask.java:1669)
    at
    org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.getMapOutput
    (ReduceTask.java:1529)
    at
    org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(R
    educeTask.java:1378)
    at
    org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTa
    sk.java:1310)
    I tried increasing the memory available using mapped.child.java.opts but that
    only helps a little. The reduce task eventually fails again. Here are some
    relevant job configuration details:

    1. The input to the mappers is about 2.5 TB (LZO compressed). The mappers
    filter out a small percentage of the input ( less than 1%).

    2. I am currently using 12 reducers and I can't increase this count by much
    to ensure availability of reduce slots for other users.

    3. mapred.child.java.opts --> -Xms512M -Xmx1536M -XX:+UseSerialGC

    4. mapred.job.shuffle.input.buffer.percent --> 0.70

    5. mapred.job.shuffle.merge.percent --> 0.66

    6. mapred.inmem.merge.threshold --> 1000

    7. I have nearly 5000 mappers which are supposed to produce LZO compressed
    outputs. The logs seem to indicate that the map outputs range between 0.3G to
    0.8GB.

    Does anything here seem amiss? I'd appreciate any input of what settings to
    try. I can try different reduced values for the input buffer percent and the
    merge percent. Given that the job runs for about 7-8 hours before crashing,
    I would like to make some informed choices if possible.

    Thanks.
    ~ Niranjan.

  • Niranjan Balasubramanian at Dec 11, 2011 at 1:32 am ⇧
    All

    Thanks for your inputs. Fortunately, our requirements for this job =
    changed, allowing me to use a combiner that ended up reducing the data =
    that was being pumped to the reducers and the problem went away.=20

    @Arun -- You are probably right that the number of reducers is rather =
    small, given the amount of data that is being reduced. We are currently =
    using the Fair scheduler and it fits our other use cases very well. The =
    newer versions of Fair scheduler appear to support limits on the reducer =
    pools. So this solution has to wait until we can make a switch.=20


    ~ Niranjan.
    On Dec 9, 2011, at 8:51 PM, Chandraprakash Bhagtani wrote:

    Hi Niranjan,

    Your issue looks similar to
    https://issues.apache.org/jira/browse/MAPREDUCE-1182 . Which hadoop version
    are you using?


    On Sat, Dec 10, 2011 at 12:17 AM, Prashant Kommireddi
    wrote:
    Arun, I faced the same issue and increasing the # of reducers fixed the
    problem.

    I was initially under the impression MR framework spills to disk if data is
    too huge to keep in memory, however on extraordinarily large reduce inputs
    this was not the case and the job failed on trying to assign the in-memory
    buffer

    private MapOutput shuffleInMemory(MapOutputLocation mapOutputLoc,
    URLConnection connection,
    InputStream input,
    int mapOutputLength,
    int compressedLength)
    throws IOException, InterruptedException {
    // Reserve ram for the map-output
    .
    .
    .
    .

    // Copy map-output into an in-memory buffer
    byte[] shuffleData = new byte[mapOutputLength];


    -Prahant Kommireddi

    On Fri, Dec 9, 2011 at 10:29 AM, Arun C Murthy <acm@hortonworks.com>
    wrote:
    Moving to mapreduce-user@, bcc common-user@. Please use project specific
    lists.

    Niranjan,

    If you average as 0.5G output per-map, it's 5000 maps *0.5G -> 2.5TB over
    12 reduces i.e. nearly 250G per reduce - compressed!

    If you think you have 4:1 compression you are doing nearly a Terabyte per
    reducer... which is way too high!

    I'd recommend you bump to somewhere along 1000 reduces to get to 2.5G
    (compressed) per reducer for your job. If your compression ratio is 2:1,
    try 500 reduces and so on.

    If you are worried about other users, use the CapacityScheduler and submit
    your job to a queue with a small capacity and max-capacity to restrict your
    job to 10 or 20 concurrent reduces at a given point.

    Arun
    On Dec 7, 2011, at 10:51 AM, Niranjan Balasubramanian wrote:

    All

    I am encountering the following out-of-memory error during the reduce
    phase of a large job.
    Map output copy failure : java.lang.OutOfMemoryError: Java heap space
    at
    org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.shuffleInMemory(ReduceTask.java:1669)
    at
    org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.getMapOutput(ReduceTask.java:1529)
    at
    org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.copyOutput(ReduceTask.java:1378)
    at
    org.apache.hadoop.mapred.ReduceTask$ReduceCopier$MapOutputCopier.run(ReduceTask.java:1310)
    I tried increasing the memory available using mapped.child.java.opts
    but
    that only helps a little. The reduce task eventually fails again. Here are
    some relevant job configuration details:
    1. The input to the mappers is about 2.5 TB (LZO compressed). The
    mappers filter out a small percentage of the input ( less than 1%).
    2. I am currently using 12 reducers and I can't increase this count by
    much to ensure availability of reduce slots for other users.
    3. mapred.child.java.opts --> -Xms512M -Xmx1536M -XX:+UseSerialGC

    4. mapred.job.shuffle.input.buffer.percent --> 0.70

    5. mapred.job.shuffle.merge.percent --> 0.66

    6. mapred.inmem.merge.threshold --> 1000

    7. I have nearly 5000 mappers which are supposed to produce LZO
    compressed outputs. The logs seem to indicate that the map outputs range
    between 0.3G to 0.8GB.
    Does anything here seem amiss? I'd appreciate any input of what
    settings
    to try. I can try different reduced values for the input buffer percent and
    the merge percent. Given that the job runs for about 7-8 hours before
    crashing, I would like to make some informed choices if possible.
    Thanks.
    ~ Niranjan.



    --
    Thanks & Regards,
    Chandra Prakash Bhagtani,
    Nokia India Pvt. Ltd.

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