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Hadoop, mail # dev - Questions about recommendation value of the "io.sort.mb" parameter


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Re: Questions about recommendation value of the "io.sort.mb" parameter
Jeff Zhang 2010-06-23, 10:08
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 <[EMAIL PROTECTED]>:
> 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 <[EMAIL PROTECTED]>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:[EMAIL PROTECTED]]
>> >Sent: Wednesday, June 23, 2010 12:27 PM
>> >To: [EMAIL PROTECTED]
>> >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