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
