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From Marc Limotte <mslimo...@gmail.com>
Subject delay before query starts processing
Date Wed, 30 Jan 2013 23:16:02 GMT
Hi,

I'm running in Amazon on an EMR cluster with hive 0.8.1.  We have a lot of
other Hadoop jobs, but only started experimenting with Hive recently.

I've been seeing a long pause after submitting a hive query and the
actually start of the hadoop job... 10 minutes or more in some cases.  I'm
wondering what's happening during this time.  Either a high level answer,
or maybe there is some logging I can turn on?

Here's some more detail.  I submit the query on the master using the hive
cli, and start to see some output right away...

Total MapReduce jobs = 2
Launching Job 1 out of 2
Number of reduce tasks not specified. Estimated from input data size: 1
In order to change the average load for a reducer (in bytes):
  set hive.exec.reducers.bytes.per.reducer=<number>
In order to limit the maximum number of reducers:
  set hive.exec.reducers.max=<number>
In order to set a constant number of reducers:
  set mapred.reduce.tasks=<number>


*[then a long delay here: 10 minutes or more... no activity in the hadoop
job tracker ui] *


… and then it continues normally ...
Starting Job = job_201301160029_0082, Tracking URL =
http://ip-xxxxxxxx.ec2.internal:9100/jobdetails.jsp?jobid=job_201301160029_0082
Kill Command = /home/hadoop/bin/hadoop job
 -Dmapred.job.tracker=xxxxxx:9001 -kill job_201301160029_0082
Hadoop job information for Stage-1: number of mappers: 2; number of
reducers: 1
2013-01-30 20:45:30,526 Stage-1 map = 0%,  reduce = 0%
…

This query is processing in the neighborhood of 500GB of data from S3.  A
couple of possibilities I thought of… perhaps someone can confirm or deny:
a) Is the data copied from S3 to HDFS during this time?
b) I have a fairly large set of libs in HIVE_AUX_JAR_PATH (around ~175
MB)-- does it have to copy these around to the tasks at this time?

Any insights appreciated.

Marc

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