Hi Todd, Sorry for bother again, could you further explain what's the 24 bytes additional overhead for each record of map output? What cost the overhead and what it is for? Thanks a lot.
Best Regards, Carp 在 2010年6月24日 上午1:49，Todd Lipcon <t...@cloudera.com>写道： > Plus there's some overhead for each record of map output. Specifically, 24 > bytes. So if you output 64MB worth of data, but each of your objects is > only > 24 bytes long itself, you need more than 128MB worth of spill space for it. > Last, the map output buffer begins spilling when it is partially full so > that more records can be collected while spill proceeds. > > 200MB io.sort.mb has enough headroom for most 64M input splits that don't > blow up the data a lot. Expanding much above 200M for most jobs doesn't buy > you much. Good news is it's easy to tell by looking at the logs to see how > many times the map tasks are spilling. If you're only spilling once, more > io.sort.mb will not help. > > -Todd > > 2010/6/23 李钰 <car...@gmail.com> > > > 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 > > > > > > > > > -- > Todd Lipcon > Software Engineer, Cloudera >
