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From Harsh J <ha...@cloudera.com>
Subject Re: instantiation of classes in MR
Date Sun, 01 Jan 2012 15:05:09 GMT
You are guaranteed one setup call for every single task attempt. This
is regardless of JVM reuse being on or off. JVM reuse will cause no
issues with what Eyal is attempting to do.

On Sun, Jan 1, 2012 at 5:49 PM, Anirudh <techie.anirudh@gmail.com> wrote:
> No problems Eyal.
>
> On  a second thought, for the JVM re-use the Mapper/Reducer instances should
> be re-used, and the setup should be called only once. This makes sense too
> as the JVM reuse is for the same job.
> You should be good with class instantiation even if the JVM reuse is
> enabled.
>
>
> On Sat, Dec 31, 2011 at 11:39 PM, Eyal Golan <egolan74@gmail.com> wrote:
>>
>> Thank you very much for the detailed explanation Anirudh.
>>
>> I think that my question about node / VM was due to some lack of knowledge
>> (I'm just starting to learn the Hadoop environment).
>> Regarding configuration of the nodes and clusters.
>> This is something that I am not doing by myself. We have a dedicated team
>> for managing the Hadoop cluster and I'll ask them.
>>
>> I think that my question should have been: How many instances of the
>> 'helper' class will be created in a single VM.
>> And, as I understand, consider I am creating the helper in the setup /
>> configure method, there would be one.
>> And as long as it's stateless, I'm good.
>>
>> Thanks again,
>>
>> Eyal
>>
>>
>>
>> Eyal Golan
>> egolan74@gmail.com
>>
>> Visit: http://jvdrums.sourceforge.net/
>> LinkedIn: http://www.linkedin.com/in/egolan74
>> Skype: egolan74
>>
>> P  Save a tree. Please don't print this e-mail unless it's really
>> necessary
>>
>>
>>
>> On Sat, Dec 31, 2011 at 1:36 PM, Anirudh <techie.anirudh@gmail.com> wrote:
>>>
>>> I just wanted to confirm where exactly you were planning to have the
>>> instantiation code, as it was not mentioned in your previous post. The
>>> location would have made difference. As you are doing it in the setup of
>>> mapper/reducer, you are good.
>>>
>>> I was referring to the Task JVM Reuse option:
>>>
>>> http://hadoop.apache.org/common/docs/current/mapred_tutorial.html#Task+JVM+Reuse
>>>
>>> It states that if the option to reuse JVM is enabled, the same Task JVM
>>> will execute multiple tasks(i.e. map/reduce). I am not sure how this is
>>> implemented, whether a new Mapper/Reducer is created for each task or they
>>> too are re-reused.
>>> If a new instance is created each time, then the mapper/reducer and  all
>>> its reference will be marked for garbage collection and you would be good.
>>> If the Mapper/Reducer instances are re-used then the setup should be
>>> called again creating another instance of your helper class.
>>>
>>> In my opinion the latter does not make sense, and the implementation
>>> would be according to the prior approach i.e. creation of a new
>>> Mapper/Reducer for each Task. But it would be interesting to check.
>>>
>>> As the classes in question are helper classes(stateless) you may not get
>>> affected in terms of functionality.
>>>
>>> I am not clear on one of your statement:
>>>
>>> How many map tasks will be created? One per split or one per VM (node)?
>>> Are you suggesting that although there would be one Mapper in the node...
>>>
>>> Have you configured your node to have a single slot for map/reduce task?
>>> If yes then there will be one Mapper/Reducer task in the node. If no there
>>> could be more than one mapper/reducer in the node depending on lots of other
>>> paramerters i.e. no of mappers/reducers slots allocated on the node, no. of
>>> input splits etc. If the node is configured to run more than one
>>> Mapper/Reducer task the scheduler may choose to run more than one task on
>>> the same node. The default is 2 Map & 2 Reduce tasks per node. And for each
>>> task a new JVM is launched unless the JVM reuse option is enabled.
>>>
>>> Thanks,
>>> Anirudh
>>>
>>>
>>> On Sat, Dec 31, 2011 at 1:28 AM, Eyal Golan <egolan74@gmail.com> wrote:
>>>>
>>>> My idea is to create that class in the setup / configure method (depends
>>>> which Mapper / Reducer I will inherit from).
>>>>
>>>> I don't understand the 'reuse' option you are referring to.
>>>> How many map tasks will be created? One per split or one per VM (node)?
>>>> Are you suggesting that although there would be one Mapper in the node,
>>>> each new operator (or reflecting) will create a new instance?
>>>> Thus making lots of that instance?
>>>>
>>>> BTW,
>>>> these helper class I want to create are of course not going to be
>>>> stateful. They are defiantly 'helper' class that have some logic.
>>>>
>>>> Thanks,
>>>>
>>>> Eyal
>>>>
>>>> Eyal Golan
>>>> egolan74@gmail.com
>>>>
>>>> Visit: http://jvdrums.sourceforge.net/
>>>> LinkedIn: http://www.linkedin.com/in/egolan74
>>>> Skype: egolan74
>>>>
>>>> P  Save a tree. Please don't print this e-mail unless it's really
>>>> necessary
>>>>
>>>>
>>>>
>>>> On Sat, Dec 31, 2011 at 6:50 AM, Anirudh <techie.anirudh@gmail.com>
>>>> wrote:
>>>>>
>>>>> Where are you creating this new class. If it is in the map function,
>>>>> then it will be create a new object for each record in the split.
>>>>>
>>>>> Also you may need to see how the JVM reuse option works. I am not too
>>>>> sure of this and you may want to look at the code. If the option for
JVM
>>>>> reuse is set, then my understanding is for every task, a new Map task
would
>>>>> be created and in that case the "new" operator will create another instance
>>>>> even if this statement is not in the map function.
>>>>>
>>>>>
>>>>> On Fri, Dec 30, 2011 at 6:22 AM, Eyal Golan <egolan74@gmail.com>
wrote:
>>>>>>
>>>>>> Great News !!
>>>>>> Thanks for the info.
>>>>>>
>>>>>> So using reflection, I can inject different implementations of
>>>>>> interfaces (services) for the mapper (or reducer).
>>>>>> And this way I can test a mapper (or reducer).
>>>>>> Just by reflecting a stub instead of a real implementation.
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>>
>>>>>>
>>>>>> Eyal Golan
>>>>>> egolan74@gmail.com
>>>>>>
>>>>>> Visit: http://jvdrums.sourceforge.net/
>>>>>> LinkedIn: http://www.linkedin.com/in/egolan74
>>>>>> Skype: egolan74
>>>>>>
>>>>>> P  Save a tree. Please don't print this e-mail unless it's really
>>>>>> necessary
>>>>>>
>>>>>>
>>>>>>
>>>>>> On Fri, Dec 30, 2011 at 2:50 PM, Harsh J <harsh@cloudera.com>
wrote:
>>>>>>>
>>>>>>> Eyal,
>>>>>>>
>>>>>>> Yes, it is right to think of each Task attempt being one individual
>>>>>>> JVM running individually on any added Node. Multiple slots would
mean
>>>>>>> multiple VMs in parallel as well. Yes, your use of reflection
to build your
>>>>>>> objects will work just fine -- its all user-side java code that
is executed.
>>>>>>>
>>>>>>> On 30-Dec-2011, at 4:42 PM, Eyal Golan wrote:
>>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> I want to understand a basic concept in MR.
>>>>>>>
>>>>>>> If a mapper creates an instance of some class (using the 'new'
>>>>>>> operator), then the created class exists ONCE in the VM of this
node.
>>>>>>> For each node.
>>>>>>> Correct?
>>>>>>>
>>>>>>> Now,
>>>>>>> what if instead of using the 'new' operator, the class is created
>>>>>>> using reflection.
>>>>>>> Is it valid in a MR?
>>>>>>> Will only one instance of the created class be existing in that
node?
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>>
>>>>>>> Eyal
>>>>>>>
>>>>>>> Eyal Golan
>>>>>>> egolan74@gmail.com
>>>>>>>
>>>>>>> Visit: http://jvdrums.sourceforge.net/
>>>>>>> LinkedIn: http://www.linkedin.com/in/egolan74
>>>>>>> Skype: egolan74
>>>>>>>
>>>>>>> P  Save a tree. Please don't print this e-mail unless it's really
>>>>>>> necessary
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>



-- 
Harsh J

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