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From Justin Pihony <justin.pih...@gmail.com>
Subject Trying to understand sc.textFile better
Date Sun, 17 May 2015 17:01:44 GMT
All,
    I am trying to understand the textFile method deeply, but I think my
lack of deep Hadoop knowledge is holding me back here. Let me lay out my
understanding and maybe you can correct anything that is incorrect

    When sc.textFile(path) is called, then defaultMinPartitions is used,
which is really just math.min(taskScheduler.defaultParallelism, 2). Let's
assume we are using the SparkDeploySchedulerBackend and this is 
    conf.getInt("spark.default.parallelism", math.max(totalCoreCount.get(),
2))
So, now let's say the default is 2, going back to the textFile, this is
passed in to HadoopRDD. The true size is determined in getPartitions() using
inputFormat.getSplits(jobConf, minPartitions). But, from what I can find,
the partitions is merely a hint and is in fact mostly ignored, so you will
probably get the total number of blocks.
OK, this fits with expectations, however what if the default is not used and
you provide a partition size that is larger than the block size. If my
research is right and the getSplits call simply ignores this parameter, then
wouldn't the provided min end up being ignored and you would still just get
the block size?

Thanks,
Justin



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