[hama-user to bcc:]

Let's not crosspost, please, it make the thread of conversation totally
opaque as to who is talking about what.

On Fri, Dec 9, 2011 at 1:42 AM, Praveen Sripati <praveensrip...@gmail.com>wrote:

> Thanks to Thomas and Avery for the response.
> > For Giraph you are quite correct, all the stuff is submitted as a MR
> job. But a full map stage is not a superstep, the whole computation is a
> done in one mapping phase.
> So a map task in MR corresponds to a computation phase in a superstep.
> Once the computation phase for a superstep is complete, the vertex output
> is stored using the defined OutputFormat, the message sent (may be) to
> another vertex and the map task is stopped. Once the barrier
> synchronization phase is complete, another set of map tasks are invoked for
> the vertices which have received a message.

In Giraph, each superstep does not lead to storage into an OutputFormat.
 The data lives all in memory from the time the first superstep starts to
the time the final superstep stops (except that for tolerance of failures,
checkpoints are stored to disk at user-specified intervals).  There is only
one set of map tasks for the Giraph job - those long-running map tasks run
possibly many supersteps.

> In a regular MR Job (not Giraph) the number of Map tasks equals to the
> number of InputSplits. But, in case of Giraph the total number of maps to
> be launched is usually more than the number of input vertices.

Number of maps > number of input vertices?  Not at all.  That would be
insane.  We want to be able to run over multi-billion vertex graphs.  We're
going to launch multiple billions of mappers?   The splitting of the data
in Giraph is very similar to in a regular MR job, divide up your input data
among the number of mappers you have, and you're off and running.

> > Where are the incoming, outgoing messages and state stored
> > Memory
> What happens if a particular node is lost in case of Hama and Giraph? Are
> the messages not persisted somewhere to be fetched later.

If nodes are lost, the system has to back up to the most recent checkpoint,
where graph state has been persisted to HDFS.  Messages are not currently
persisted, but the state at which the graph was in to produce any messages

> > In Giraph, vertices can move around workers between supersteps.  A
> vertex will run on the worker that it is assigned to.
> Is data locality considered while moving vertices around workers in Giraph?

Data is all in memory, and typical graph algorithms are basically sending
roughly the size of the entire graph (number of total edges) out over
distributed RPC in any given superstep, so shuffling the graph around by
RPC is not much more to do.

> > As you can see, you could write a MapReduce Engine with BSP on top of
> Apache Hama.
> It's being the done other way, BSP is implemented in Giraph using Hadoop.

I'll let the Hama people explain to you about how one would implement MR on
top of Hama.  You are correct that in Giraph, the Hadoop
JobTracker/TaskTracker and HDFS are used as substrate to help implement BSP
(although I would not say that "MR" is being used to implement BSP, as
there is no MR going on in Giraph).


> Praveen
> On Fri, Dec 9, 2011 at 12:51 PM, Avery Ching <ach...@apache.org> wrote:
>>  Hi Praveen,
>> Answers inline.  Hope that helps!
>> Avery
>> On 12/8/11 10:16 PM, Praveen Sripati wrote:
>> Hi,
>> I know about MapReduce/Hadoop and trying to get myself around
>> BSP/Hama-Giraph by comparing MR and BSP.
>> - Map Phase in MR is similar to Computation Phase in BSP. BSP allows for
>> process to exchange data in the communication phase, but there is no
>> communication between the mappers in the Map Phase. Though the data flows
>> from Map tasks to Reducer tasks. Please correct me if I am wrong. Any other
>> significant differences?
>> I suppose you can think of it that way.  I like to compare a BSP
>> superstep to a MapReduce job since it's computation and communication.
>> - After going through the documentation for Hama and Giraph, noticed that
>> they both use Hadoop as the underlying framework. In both Hama and Giraph
>> an MR Job is submitted. Does each superstep in BSP correspond to a Job in
>> MR? Where are the incoming, outgoing messages and state stored - HDFS or
>> HBase or Local or pluggable?
>>  My understanding of Hama is that they have their own BSP framework.
>> Giraph can be run on a Hadoop installation, it does not have its own
>> computational framework.  A Giraph job is submitted to a Hadoop
>> installation as a Map-only job.  Hama will have its own BSP lauching
>> framework.
>> In Giraph, the state is stored all in memory.  Graphs are loaded/stored
>> through VertexInputFormat/VertexOutputFormat (very similar to Hadoop).  You
>> could implement your own VertexInputFormat/VertexOutputFormat to use HDFS,
>> HBase, etc. as your graph stable storage.
>> - If a Vertex is deactivated and again activated after receiving a
>> message, does is run on the same node or a different node in the cluster?
>>  In Giraph, vertices can move around workers between supersteps.  A
>> vertex will run on the worker that it is assigned to.
>> Regards,
>> Praveen

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