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From "Mungre,Surbhi" <Surbhi.Mun...@Cerner.com>
Subject Re: Same processing in two m/r jobs
Date Tue, 12 Nov 2013 17:31:32 GMT
Hey Josh,
Thanks for the reply! I think we will be able to get around this issue by materializing the
output of union of S2 and S3.
However, the DAG shows that the first job is still writing output of S1 to the disk. Out of
curiosity, how the planner decides to write output of S1 to the disk instead of writing the
output of union of S2 and S3?

-Surbhi

From: Josh Wills <jwills@cloudera.com<mailto:jwills@cloudera.com>>
Reply-To: "user@crunch.apache.org<mailto:user@crunch.apache.org>" <user@crunch.apache.org<mailto:user@crunch.apache.org>>
Date: Tuesday, November 12, 2013 10:26 AM
To: "user@crunch.apache.org<mailto:user@crunch.apache.org>" <user@crunch.apache.org<mailto:user@crunch.apache.org>>
Subject: Re: Same processing in two m/r jobs

Hey Surbhi,

The planner is trying to minimize the amount of data it writes to disk at the end of the first
job; it doesn't usually worry so much about re-running the same computation in two different
jobs if it means that less data will be written to disk overall, since most MR jobs aren't
CPU bound.

While that's often a useful heuristic, there are many cases where it isn't true, and this
sounds like one of them. My advice would be to materialize the output of the union of S2 and
S3, at which point the planner should run the processing of S2 and S3 once at the end of job
1, and then pick up that materialized output for grouping in job 2.

Best,
Josh


On Mon, Nov 11, 2013 at 8:30 PM, Mungre,Surbhi <Surbhi.Mungre@cerner.com<mailto:Surbhi.Mungre@cerner.com>>
wrote:
Background:
We have a crunch pipeline which is used to normalize and standardize some entities represented
as Avro. In our pipeline, we also capture some context information about the errors and warnings
which we encounter during our processing. We pass a pair of context information and Avro entities
in our pipeline. At the end of the pipeline, the context information is written to HDFS and
Avro entities are written to HFiles.

Problem:
When we were trying to analyze DAG for our crunch pipeline we noticed that same processing
is done in two m/r jobs. Once it is done to capture context information and second time it
is done to generate HFiles. I wrote a test which replicates this issue with a simple example.
The test and a DAG created from this test are attached with the post. It is clear from the
DAG that S2 and S3 are processed twice. I am not sure why this processing is done twice and
if there is any way to avoid this behavior.

Surbhi Mungre


Software Engineer
www.cerner.com<http://www.cerner.com>



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