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Pig, mail # dev - Number of mappers in MRCompiler


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Re: Number of mappers in MRCompiler
Alan Gates 2012-08-23, 22:20
Sorry for the very slow response, but here it is, hopefully better late than never.

On Jul 25, 2012, at 4:28 PM, Prasanth J wrote:

> Thanks Alan.
> The requirement for me is that I want to load N number of samples based on the input file size and perform naive cube computation to determine the large groups that will not fit in reducer's memory. I need to know the exact number of samples for calculating the partition factor for large groups.
> Currently I am using RandomSampleLoader to load 1000 tuples from each mapper. Without knowing the number of mappers I will not be able to find the exact number of samples loaded. Also RandomSampleLoader doesn't attach any special marker (as in PoissonSampleLoader) tuples which tells the number of samples loaded.
> Is there any other way to know the exact number of samples loaded?
Not that I know of.

>
> By analyzing the MR plans of order-by and skewed-join, it seems like the entire dataset is copied to a temp file and then SampleLoaders use the temp file to load samples. Is there any specific reason for this redundant copy? Is it because SampleLoaders can only use pig's internal i/o format?
Partly, but also because it allows any operators that need to run before the sample (like project or filter) to be placed in the pipeline.

Alan.