Hi Dean, Thanks for your reply. If I don't set the number of reducers in the 1st run , the number of reducers will be much smaller and the performance will be worse. The total output file size is about 200MB, I see that many reduce output files are empty, only 10 of them have data.
Another question is that , is there any documentation about the job specific parameters of MapReduce and Hive? 2013/6/29 Dean Wampler <deanwamp...@gmail.com> > What happens if you don't set the number of reducers in the 1st run? How > many reducers are executed. If it's a much smaller number, the extra > overhead could matter. Another clue is the size of the files the first run > produced, i.e., do you have 30 small (much less than a block size) files? > > On Sat, Jun 29, 2013 at 12:27 AM, Felix.徐 <ygnhz...@gmail.com> wrote: > >> Hi Stephen, >> >> My query is actually more complex , hive will generate 2 mapreduces, >> in the first solution , it runs 17 mappers / 30 reducers and 10 mappers / >> 30 reducers (reducer num is set manually) >> in the second solution , it runs 6 mappers / 1 reducer and 4 mappers / 1 >> reducers for each partition >> >> I do not know whether they could achieve the same performance if the >> reducers num is set properly. >> >> >> 2013/6/29 Stephen Sprague <sprag...@gmail.com> >> >>> great question. your parallelization seems to trump hadoop's. I >>> guess i'd ask what are the _total_ number of Mappers and Reducers that run >>> on your cluster for these two scenarios? I'd be curious if there are the >>> same. >>> >>> >>> >>> >>> On Fri, Jun 28, 2013 at 8:40 AM, Felix.徐 <ygnhz...@gmail.com> wrote: >>> >>>> Hi all, >>>> >>>> Here is the scenario, suppose I have 2 tables A and B, I would like to >>>> perform a simple join on them, >>>> >>>> We can do it like this: >>>> >>>> INSERT OVERWRITE TABLE C >>>> SELECT .... FROM A JOIN B on A.id=B.id >>>> >>>> In order to speed up this query since table A and B have lots of data, >>>> another approach is : >>>> >>>> Say I partition table A and B into 10 partitions respectively, and >>>> write the query like this >>>> >>>> INSERT OVERWRITE TABLE C PARTITION(pid=1) >>>> SELECT .... FROM A JOIN B on A.id=B.id WHERE A.pid=1 AND B.pid=1 >>>> >>>> then I run this query 10 times concurrently (pid ranges from 1 to 10) >>>> >>>> And my question is that , in my observation of some more complex >>>> queries, the second solution is about 15% faster than the first solution, >>>> is it simply because the setting of reducer num is not optimal? >>>> If the resource is not a limit and it is possible to set the proper >>>> reducer nums in the first solution , can they achieve the same performance? >>>> Is there any other fact that can cause performance difference between >>>> them(non-partition VS partition+concurrent) besides the job parameter >>>> issues? >>>> >>>> Thanks! >>>> >>> >>> >> > > > -- > Dean Wampler, Ph.D. > @deanwampler > http://polyglotprogramming.com
