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From Hemanth Yamijala <yhema...@thoughtworks.com>
Subject Re: Question about the task assignment strategy
Date Tue, 11 Sep 2012 14:27:02 GMT

Task assignment takes data locality into account first and not block
sequence. In hadoop, tasktrackers ask the jobtracker to be assigned tasks.
When such a request comes to the jobtracker, it will try to look for an
unassigned task which needs data that is close to the tasktracker and will
assign it.


On Tue, Sep 11, 2012 at 6:31 PM, Hiroyuki Yamada <mogwaing@gmail.com> wrote:

> Hi,
> I want to make sure my understanding about task assignment in hadoop
> is correct or not.
> When scanning a file with multiple tasktrackers,
> I am wondering how a task is assigned to each tasktracker .
> Is it based on the block sequence or data locality ?
> Let me explain my question by example.
> There is a file which composed of 10 blocks (block1 to block10), and
> block1 is the beginning of the file and block10 is the tail of the file.
> When scanning the file with 3 tasktrackers (tt1 to tt3),
> I am wondering if
> task assignment is based on the block sequence like
> first tt1 takes block1 and tt2 takes block2 and tt3 takes block3 and
> tt1 takes block4 and so on
> or
> task assignment is based on the task(data) locality like
> first tt1 takes block2(because it's located in the local) and tt2
> takes block1 (because it's located in the local) and
> tt3 takes block 4(because it's located in the local) and so on.
> As far as I experienced and the definitive guide book says,
> I think that the first case is the task assignment strategy.
> (and if there are many replicas, closest one is picked.)
> Is this right ?
> If this is right, is there any way to do like the second case
> with the current implementation ?
> Thanks,
> Hiroyuki

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