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From Harsh J <ha...@cloudera.com>
Subject Re: Fault tolerance and Speculative Execution
Date Thu, 18 Jul 2013 17:10:31 GMT
What you describe in the first paragraph is not true.

Speculative execution API toggles are listed in the documentation:
and in the mapred-default page in property form:
http://hadoop.apache.org/docs/stable/mapred-default.html. Speculative
execution is enabled by default.

On Thu, Jul 18, 2013 at 10:32 PM, Sundeep Kambhampati
<kambhamp@cse.ohio-state.edu> wrote:
> Hi all,
> Is it true that Hadoop 'always' starts same map tasks multiple times in
> order to be fault tolerant. i.e. same task is launched on several machines
> so that even if a node fails then same task would be available on other
> node. And in case no node fails redundant task that finishes late is killed.
> If it is true how can I change that configuration for Hadoop to do it or not
> do it.
> Speculative execution on the other hand does what I explained above
> (redundant map tasks) but only after all the map tasks are scheduled and if
> some nodes are free it starts redundant map tasks for those which are
> running slow. Is it always true? How do change this configuration
> enable/disable.
> I am using Hadoop-1.1.2 incase version matters.
> I really appreciate if someone could help me with this. Thank you.
> Regards
> Sundeep

Harsh J

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