Hi Jeff, Thanks for your quick reply. Seems my thinking is stuck on the job style I'm running. Now I'm much clearer about it.
Best Regards, Carp 2010/6/23 Jeff Zhang <zjf...@gmail.com> > Hi 李钰 > > The size of map output depends on your Mapper class. The Mapper class > will do processing on the input data. > > > > 2010/6/23 李钰 <car...@gmail.com>: > > Hi Sriguru, > > > > Thanks a lot for your comments and suggestions! > > Here I still have some questions: since map mainly do data preparation, > > say split input data into KVPs, sort and partition before spill, would > the > > size of map output KVPs be much larger than the input data size? If not, > > since one map task deals with one input split, and one input split is > > usually 64M, the map KVPs size would be proximately 64M. Could you please > > give me some example on map output much larger than the input split? It > > really confuse me for some time, thanks. > > > > Others, > > > > Also badly need your help if you know about this, thanks. > > > > Best Regards, > > Carp > > > > 在 2010年6月23日 下午5:11，Srigurunath Chakravarthi <srig...@yahoo-inc.com>写道： > > > >> Hi Carp, > >> Your assumption is right that this is a per-map-task setting. > >> However, this buffer stores map output KVPs, not input. Therefore the > >> optimal value depends on how much data your map task is generating. > >> > >> If your output per map is greater than io.sort.mb, these rules of thumb > >> that could work for you: > >> > >> 1) Increase max heap of map tasks to use RAM better, but not hit swap. > >> 2) Set io.sort.mb to ~70% of heap. > >> > >> Overall, causing extra "spills" (because of insufficient io.sort.mb) is > >> much better than risking swapping (by setting io.sort.mb and heap too > >> large), in terms of relative performance penalty you will pay. > >> > >> Cheers, > >> Sriguru > >> > >> >-----Original Message----- > >> >From: 李钰 [mailto:car...@gmail.com] > >> >Sent: Wednesday, June 23, 2010 12:27 PM > >> >To: common-dev@hadoop.apache.org > >> >Subject: Questions about recommendation value of the "io.sort.mb" > >> >parameter > >> > > >> >Dear all, > >> > > >> >Here I've got a question about the "io.sort.mb" parameter. We can find > >> >material from Yahoo! or Cloudera which recommend setting this value to > >> >200 > >> >if the job scale is large, but I'm confused about this. As I know, > >> >the tasktracker will launch a child-JVM for each task, and > >> >“*io.sort.mb*” > >> >presents the buffer size in memory inside *one map task child-JVM*, the > >> >default value 100MB should be large enough because the input split of > >> >one > >> >map task is usually 64MB, as large as the block size we usually set. > >> >Then > >> >why the recommendation of “*io.sort.mb*” is 200MB for large jobs (and > >> >it > >> >really works)? How could the job size affect the procedure? > >> >Is there any fault here of my understanding? Any comment/suggestion > >> >will be > >> >highly valued, thanks in advance. > >> > > >> >Best Regards, > >> >Carp > >> > > > > > > -- > Best Regards > > Jeff Zhang >
