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From Michael Segel <michael_se...@hotmail.com>
Subject Re: Hadoop counter
Date Fri, 19 Oct 2012 14:35:34 GMT
As I understand it... each Task has its own counters and are independently updated. As they
report back to the JT, they update the counter(s)' status.
The JT then will aggregate them. 

In terms of performance, Counters take up some memory in the JT so while its OK to use them,
if you abuse them, you can run in to issues. 
As to limits... I guess that will depend on the amount of memory on the JT machine, the size
of the cluster (Number of TT) and the number of counters. 

In terms of global accessibility... Maybe.

The reason I say maybe is that I'm not sure by what you mean by globally accessible. 
If a task creates and implements a dynamic counter... I know that it will eventually be reflected
in the JT. However, I do not believe that a separate Task could connect with the JT and see
if the counter exists or if it could get a value or even an accurate value since the updates
are asynchronous.  Not to mention that I don't believe that the counters are aggregated until
the job ends. It would make sense that the JT maintains a unique counter for each task until
the tasks complete. (If a task fails, it would have to delete the counters so that when the
task is restarted the correct count is maintained. )  Note, I haven't looked at the source
code so I am probably wrong. 

HTH
Mike
On Oct 19, 2012, at 5:50 AM, Lin Ma <linlma@gmail.com> wrote:

> Hi guys,
> 
> I have some quick questions regarding to Hadoop counter,
> 
> Hadoop counter (customer defined) is global accessible (for both read and write) for
all Mappers and Reducers in a job?
> What is the performance and best practices of using Hadoop counters? I am not sure if
using Hadoop counters too heavy, there will be performance downgrade to the whole job?
> regards,
> Lin


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