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Pig, mail # dev - Number of mappers in MRCompiler


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Re: Number of mappers in MRCompiler
Prasanth J 2012-08-24, 03:23
Oh yeah.. This question is not related to our cube sampling stuff that we discussed.. wanted to know the reason behind that just out of curiosity :)
Thanks
-- Prasanth

On Aug 23, 2012, at 11:20 PM, Dmitriy Ryaboy <[EMAIL PROTECTED]> wrote:

> I think we decided to instead stub in a special loader that reads a
> few records from each underlying split, in a single mapper (by using a
> single wrapping split), right?
>
> On Thu, Aug 23, 2012 at 7:55 PM, Prasanth J <[EMAIL PROTECTED]> wrote:
>> I see. Thanks Alan for your reply.
>> Also one more question that I posted earlier was
>>
>> I used RandomSampleLoader and specified a sample size of 100. The number of map tasks that are executed is 110. So I am expecting total samples that are received on the reducer to be 110*100 = 11000 but its always more than the expected value. The actual received tuples is between 14000 to 15000. I am not sure if its a bug or if I am missing something. Is it an expected behavior?
>>
>> Thanks
>> -- Prasanth
>>
>> On Aug 23, 2012, at 6:20 PM, Alan Gates <[EMAIL PROTECTED]> wrote:
>>
>>> 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.
>>>
>>