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From Michael Armbrust <mich...@databricks.com>
Subject Re: unintended consequence of using coalesce operation
Date Wed, 30 Sep 2015 01:00:12 GMT
coalesce is generally to avoid launching too many tasks, on a bunch of
small files.  As a result, the goal is to reduce parallelism (when the
overhead of that parallelism is more costly than the gain).  You are
correct that in you case repartition sounds like a better choice.

On Tue, Sep 29, 2015 at 4:33 PM, Lan Jiang <ljiang2@gmail.com> wrote:

> Hi, there
>
> I ran into an issue when using Spark (v 1.3) to load avro file through
> Spark SQL. The code sample is below
>
> val df = sqlContext.load(“path-to-avro-file","com.databricks.spark.avro”)
> val myrdd = df.select(“Key", “Name", “binaryfield").rdd
> val results = myrdd.map(...)
> val finalResults = results.filter(...)
> finalResults.*coalesce(1)*.toDF().saveAsParquetFile(“path-to-parquet”)
>
> The avro file 645M. The HDFS block size is 128M. Thus the total is 5 HDFS
> blocks, which means there should be 5 partitions. Please note that I use
> coalesce because I expect the previous filter transformation should filter
> out almost all the data and I would like to write to 1 single parquet file.
>
> YARN cluster has 3 datanodes. I use the below configuration for spark
> submit
>
> spark-submit —class <myclass> —num-executors 3 —executor-cores 2
> —executor-memory 8g —master yarn-client mytest.jar
>
> I do see 3 executors being created, one on each data/worker node. However,
> there is only one task running within the cluster.  After I remove the
> coalesce(1) call from the codes, I can see 5 tasks generates, spreading
> across 3 executors.  I was surprised by the result. coalesce usually is
> thought to be a better choice than repartition operation when reducing the
> partition numbers. However, in the case, it causes performance issue
> because Spark only creates one task because the final partition number was
> coalesced to 1.  Thus there is only one thread reading HDFS files instead
> of 5.
>
> Is my understanding correct? In this case, I think repartition is a better
> choice than coalesce.
>
> Lan
>
>
>
>

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