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From Owen O'Malley <o...@yahoo-inc.com>
Subject Re: RandomWriter/Sort sanity check
Date Wed, 09 May 2007 20:39:40 GMT

On May 9, 2007, at 12:18 PM, Steve Schlosser wrote:

> To start with, I
> modified the benchmark to just write out 1GB of data per node, rather
> than the default 10GB, since I don't have a whole lot of disk capacity
> at the moment.

You don't actually need to change code to do that. You could also  
create a config file with test.randomwriter.maps_per_host or  
test.randomwrite.bytes_per_map set to a lower number than the defaults.

> 12 nodes gets me a 21X speedup over 1 node, and 13 nodes gets me a 33x
> speedup over 1 node.  This seems too good to be true - what could
> Hadoop be doing?  For the 13-node runs, there are only ever 13 reduce
> tasks, and mapred.tasktracker.tasks.maximum is set to 1.  Can anyone
> shed some light?

I'm not sure what is going on, but it is probably related to whether  
the data is kept in memory or spills to disk. The framework tries to  
keep as much data in RAM as possible to avoid trips to disk.

> Just to make sure I understand, Sort itself does nothing but force
> Hadoop to partition the input data (1GB per node in my case) and sort
> it.  Should I think of the sort as being part of the Map phase or the
> Reduce phase?  That is, is there one sort per node?  One sort per Map
> task?  One sort per Reduce task?

*Laugh* The answer is more complicated than that. In Hadoop 0.1, the  
sort was all done on the reduce side. Currently the map outputs are  
sorted on the map side. The reduce merges the various map outputs.

-- Owen

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