Comparing Riak MapReduce and Hadoop MapReduce

Jeremiah Peschka jeremiah.peschka at gmail.com
Mon Jul 22 13:21:27 EDT 2013


For JavaScript the number of reducers is configured in the app.config file
on each node with the reduce_js_vm_count property.

---
Jeremiah Peschka - Founder, Brent Ozar Unlimited
MCITP: SQL Server 2008, MVP
Cloudera Certified Developer for Apache Hadoop


On Mon, Jul 22, 2013 at 8:07 AM, Xiaoming Gao <mkobie at gmail.com> wrote:

> Thanks for the clarification, Jeremiah!
>
> One last question: how should I configure the MR job to have multiple
> reducer processes on a single node?
>
> Regards,
> Xiaoming
>
>
> On Mon, Jul 22, 2013 at 1:33 AM, Jeremiah Peschka [via Riak Users] <[hidden
> email] <http://user/SendEmail.jtp?type=node&node=4028486&i=0>> wrote:
>
>> Ah, yeah, I'm mistaken about search partitioning. The docs are correct.
>>
>> I have no idea how the scheduling works.
>>
>> If I had to guess, I would guess that it is a streaming operation.
>>
>> --
>> Jeremiah Peschka - Founder, Brent Ozar Unlimited
>> MCITP: SQL Server 2008, MVP
>> Cloudera Certified Developer for Apache Hadoop
>>
>> On Jul 21, 2013, at 10:08 PM, Xiaoming Gao <[hidden email]<http://user/SendEmail.jtp?type=node&node=4028477&i=0>>
>> wrote:
>>
>> Thanks a lot, Jeremiah! Your answers really help clarify the issues.
>>
>> Just one more question, by "document-based indices", do you mean
>> document-based partitioning for the indices? Because what I found in the
>> online document
>> http://docs.basho.com/riak/latest/dev/advanced/search/#Search-KV-and-MapReduceis "Search
>> uses term-based partitioning – also known as a global index." I am not sure
>> if the implementation has changed for the latest version of Riak, but if
>> term-based partitioning is used, does that mean Riak will only schedule the
>> mappers after the whole list of <bucket, key> pair is returned from the
>> index?
>>
>> Thanks,
>> Xiaoming
>>
>>
>> On Sun, Jul 21, 2013 at 11:20 PM, Jeremiah Peschka [via Riak Users] <[hidden
>> email] <http://user/SendEmail.jtp?type=node&node=4028476&i=0>> wrote:
>>
>>> Responses inline. Hopefully they shed some light on the subject.
>>>
>>> ---
>>> Jeremiah Peschka - Founder, Brent Ozar Unlimited
>>> MCITP: SQL Server 2008, MVP
>>> Cloudera Certified Developer for Apache Hadoop
>>>
>>>
>>> On Fri, Jul 19, 2013 at 5:07 PM, Xiaoming Gao <[hidden email]<http://user/SendEmail.jtp?type=node&node=4028474&i=0>
>>> > wrote:
>>>
>>>> Hi everyone,
>>>>
>>>> I am trying to learn about Riak MapReduce and comparing it with Hadoop
>>>> MapReduce, and there are some details that I am interested in but not
>>>> covered in the online documents. So hopefully we can get some help here
>>>> about the following questions? Thanks in advance!
>>>>
>>>
>>> They're not at all similar. Hadoop MR is optimized for sequential data
>>> processing in large batches. Riak MR works better when you think of it like
>>> a multi-processing engine - you can perform work across a matching set of
>>> items and that work will be distributed across the cluster during map
>>> phases.
>>>
>>> Take a look at this thread for a bit of discussion about when you should
>>> use Riak MapReduce: http://markmail.org/message/qpoilvmm635inb5v
>>>
>>> Or, if you want to, you can run a Riak MR job across an entire bucket,
>>> which really is like scanning every table in an RDBMS while looking for
>>> rows from a single table. MR jobs run with an R of 1. So, at least there's
>>> that.
>>>
>>>
>>>> 1. For a given MapReduce request (or to say, job), how does Riak decide
>>>> how
>>>> many mappers to use for the job? For example, if I have 8 nodes and my
>>>> data
>>>> are distributed across all nodes with an "N" value of 2, will I have 4
>>>> mappers running on 4 nodes concurrently? Is it possible to have multiple
>>>> mappers (e.g., 4 or even 6) for the same MR job running on each node
>>>> (for
>>>> better processing speed)?
>>>>
>>>
>>> To the best of my recollection, this will be based on either:
>>>
>>> 1) If you're using JavaScript MR jobs, the number of mappers and
>>> reducers is controlled by the the map_js_vm_count and reduce_js_vm_count
>>> settings from each node's app.config file.
>>> 2) If you're using Erlang: magic. This will be handled by the Erlang VM
>>> and is based on number of processors and your overall Erlang VM
>>> configuration.
>>>
>>>
>>>>
>>>> 2. If I run a MapReduce job over the results of a Riak Search query, how
>>>> does Riak schedule the mappers based on the search results?
>>>>
>>>
>>> Riak Search uses document-based indices - search will query every node
>>> in the cluster. Map phases happen and then results are then streamed to the
>>> reducer.
>>>
>>>
>>>>
>>>> 3. How does Riak handle intermediate data generated by mappers?
>>>> Specifically:
>>>> (1) In Hadoop MapReduce, the output of mappers are <key, value> pairs,
>>>> and
>>>> the output from all mappers are first grouped based on keys, and then
>>>> handed
>>>> over to the reducer. Does Riak do similar grouping of intermediate data?
>>>>
>>>
>>> The only reason for the intermediate grouping/scratch work in Hadoop MR
>>> jobs is to deal with multiple reducers. Although, I'm not entirely sure how
>>> this works in Riak, my suspicion is that data is streamed across the wire
>>> after the data is read from disk.
>>>
>>>
>>>>
>>>> (2) How are mapper outputs transmitted to the reducer? Does Riak use
>>>> local
>>>> disks on the mapper nodes or reducer nodes to store the intermediate
>>>> data
>>>> temporarily?
>>>
>>>
>>> Since large MR jobs can cause out of memory errors, you can bet good
>>> money that the answer is "no".
>>>
>>>
>>>>
>>>> 4. According to the document
>>>>
>>>> http://docs.basho.com/riak/latest/dev/advanced/mapreduce/#How-Phases-Work,
>>>> each MR job only schedules one reducer, which runs on the coordinate
>>>> node.
>>>> Is there any way to configure a MR job to use multiple reducers?
>>>>
>>>
>>> Using Riak MR, there's no way to create a job that runs reducers on
>>> multiple nodes. You can have multiple reducer processes on a single node,
>>> but not reducers on multiple nodes.
>>>
>>>
>>>>
>>>> Best regards,
>>>> Xiaoming
>>>>
>>>>
>>>>
>>>> --
>>>> View this message in context:
>>>> http://riak-users.197444.n3.nabble.com/Comparing-Riak-MapReduce-and-Hadoop-MapReduce-tp4028454.html
>>>> Sent from the Riak Users mailing list archive at Nabble.com.
>>>>
>>>>
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