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Date: Wed, 02 May 2012 15:30:22 -0700
From: Avery Ching <aching@apache.org>
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To: giraph-user@incubator.apache.org
Subject: Re: Possible bug when resetting aggregators ? (and missing
 documentation)
References: <A6A13C38-90B2-494C-934D-E884D3AF861C@deri.org>
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I think you're right that the javadoc isn't specific enough.

    * Use a registered aggregator in current superstep.
    * Even when the same aggregator should be used in the next
    * superstep, useAggregator needs to be called at the beginning
    * of that superstep in preSuperstep().
    *
    * @param name Name of aggregator
    * @return boolean (false when not registered)
    */
   boolean useAggregator(String name);

This should be augmented to say that none of the Aggregator methods 
should be called until this method is invoke.  Feel free to file a JIRA 
and fix.  Thanks!

If you would like to, please feel free to add Aggregator documentation 
to https://cwiki.apache.org/confluence/display/GIRAPH/Index

Avery

On 5/2/12 12:15 PM, Benjamin Heitmann wrote:
> Hello,
>
> I had to use aggregators for various statistic reporting tasks,
> and I noticed that the aggregator operations need to be used in a very specific squence,
> especially when the aggregator is getting a reset between supersteps.
>
> I found that the sequence described in RandomMessageBenchmark (in the org.apache.giraph.benchmark package)
> results in consistent counts for one aggregator across all workers.
> The most important thing, seems to be to call the reset method setAggregatedValue() in preSuperstep() of the WorkerContext class,
> before calling this.useAggregator().
>
> If I called the reset method in postSuperstep(), then every worker reported a different value for the aggregator.
>
> However, the aggregator which gets the reset between supersteps, still is wrong.
>
> I know this, because a second aggregator counts the same thing, and reports it after each superstep,
> without getting a reset.
>
> Is this a known issue ? Should I file a bug report on it ?
>
>
> In addition, it would be great to document correct usage of the aggregators somewhere.
> Even just in the javadoc of the aggregator interface might be enough.
>
> Should I try to add some documentation to the aggregator interface?
> (org.apache.giraph.graph.Aggregator.java)
> Then the committers can correct me if that documentation is wrong, I guess.