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From Claudio Martella <claudio.marte...@gmail.com>
Subject Re: Basic questions about Giraph internals
Date Thu, 06 Feb 2014 12:06:09 GMT
On Thu, Feb 6, 2014 at 12:15 PM, Alexander Frolov
<alexndr.frolov@gmail.com>wrote:

>
>
>
> On Thu, Feb 6, 2014 at 3:00 PM, Claudio Martella <
> claudio.martella@gmail.com> wrote:
>
>>
>>
>>
>> On Thu, Feb 6, 2014 at 11:56 AM, Alexander Frolov <
>> alexndr.frolov@gmail.com> wrote:
>>
>>> Hi Claudio,
>>>
>>> thank you.
>>>
>>> If I understood correctly, mapper and mapper task is the same thing.
>>>
>>
>> More or less. A mapper is a functional element of the programming model,
>> while the mapper task is the task that executes the mapper function on the
>> records.
>>
>
> Ok, I see. Then mapred.tasktracker.map.tasks.maximum is a maximum number
> of Workers [or Workers + Master] which will be created at the same node.
>
> That is if I have 8 node cluster
> with mapred.tasktracker.map.tasks.maximum=4, then I can run up to 31
> Workers + 1 Master.
>
> Is it correct?
>

That is correct. However, if you have total control over your cluster, you
may want to run one worker per node (hence setting the max number of map
tasks per machine to 1), and use multiple threads (input, compute, output).
This is going to make better use of resources.


>
>
>>
>>>
>>>
>>> On Thu, Feb 6, 2014 at 2:28 PM, Claudio Martella <
>>> claudio.martella@gmail.com> wrote:
>>>
>>>> Hi Alex,
>>>>
>>>> answers are inline.
>>>>
>>>>
>>>> On Thu, Feb 6, 2014 at 11:22 AM, Alexander Frolov <
>>>> alexndr.frolov@gmail.com> wrote:
>>>>
>>>>> Hi, folks!
>>>>>
>>>>> I have started small research of Giraph framework and I have not much
>>>>> experience with Giraph and Hadoop :-(.
>>>>>
>>>>> I would like to ask several questions about how things are working in
>>>>> Giraph which are not straightforward for me. I am trying to use the sources
>>>>> but sometimes it is not too easy ;-)
>>>>>
>>>>> So here they are:
>>>>>
>>>>> 1) How Workers are assigned to TaskTrackers?
>>>>>
>>>>
>>>> Each worker is a mapper, and mapper tasks are assigned to tasktrackers
>>>> by the jobtracker.
>>>>
>>>
>>> That is each Worker is created at the beginning of superstep and then
>>> dies. In the next superstep all Workers are created again. Is it correct?
>>>
>>
>> Nope. The workers are created at the beginning of the computation, and
>> destroyed at the end of the computation. A computation is persistent
>> throughout the computation.
>>
>>
>>>
>>>
>>>> There's no control by Giraph there, and because Giraph doesn't need
>>>> data-locality like Mapreduce does, basically nothing is done.
>>>>
>>>
>>> This is important for me. So Giraph Worker (a.k.a Hadoop mapper) fetches
>>> vertex with corresponding index from the HDFS and perform computation. What
>>> does it do next with it? As I understood Giraph is fully in-memory
>>> framework and in the next superstep this vertex should be fetched from the
>>> memory by the same Worker. Where the vertices are stored between
>>> supersteps? In HDFS or in memory?
>>>
>>
>> As I said, the workers are persistent (in-memory) between supersteps, so
>> they keep everything in memory.
>>
>
> Ok.
>
> Is there any means to see assignment of Workers to TaskTrackers during or
> after the computation?
>

The jobtracker http interface will show you the mapper running, hence i'd
check there


>
> And is there any means to see assignment of vertices to Workers (as
> distribution function, histogram etc.)?
>

You can check the worker logs, I think the information should be there.


>
>
>
>>
>>>
>>>
>>>>
>>>>>
>>>>> 2) How vertices are assigned to Workers? Does it depend on
>>>>> distribution of input file on DataNodes? Is there available any choice
of
>>>>> distribution politics or no?
>>>>>
>>>>
>>>> In the default scheme, vertices are assigned through modulo hash
>>>> partitioning. Given k workers, vertex v is assigned to worker i according
>>>> to hash(v) % k = i.
>>>>
>>>
>>>>
>>>>>
>>>>> 3) How Workers and Map tasks are related to each other? (1:1)? (n:1)?
>>>>> (1:n)?
>>>>>
>>>>
>>>> It's 1:1. Each worker is implemented by a mapper task. The master is
>>>> usually (but does not need to) implemented by an additional mapper
>>>>
>>> .
>>>>
>>>>
>>>>>
>>>>> 4) Can Workers migrate from one TaskTracker to the other?
>>>>>
>>>>
>>>> Workers does not migrate. A Giraph computation is not dynamic wrt to
>>>> assignment and size of the tasks.
>>>>
>>>
>>>>
>>>>>
>>>>> 5) What is the best way to monitor Giraph app execution (progress,
>>>>> worker assignment, load balancing etc.)?
>>>>>
>>>>
>>>> Just like you would for a standard Mapreduce job. Go to the job page on
>>>> the jobtracker http page.
>>>>
>>>>
>>>>>
>>>>> I think this is all for the moment. Thank you.
>>>>>
>>>>> Testbed description:
>>>>> Hardware: 8 node dual-CPU cluster with IB FDR.
>>>>> Giraph: release-1.0.0-RC2-152-g585511f
>>>>> Hadoop: hadoop-0.20.203.0, hadoop-rdma-0.9.8
>>>>>
>>>>> Best,
>>>>>    Alex
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>>    Claudio Martella
>>>>
>>>>
>>>
>>>
>>
>>
>> --
>>    Claudio Martella
>>
>>
>
>


-- 
   Claudio Martella

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