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 <mko...@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. >>>> >>>> >>>> _______________________________________________ >>>> riak-users mailing list >>>> [hidden email] <http://user/SendEmail.jtp?type=node&node=4028474&i=1> >>>> http://lists.basho.com/mailman/listinfo/riak-users_lists.basho.com >>>> >>> >>> >>> _______________________________________________ >>> riak-users mailing list >>> [hidden email] <http://user/SendEmail.jtp?type=node&node=4028474&i=2> >>> http://lists.basho.com/mailman/listinfo/riak-users_lists.basho.com >>> >>> >>> ------------------------------ >>> If you reply to this email, your message will be added to the >>> discussion below: >>> >>> http://riak-users.197444.n3.nabble.com/Comparing-Riak-MapReduce-and-Hadoop-MapReduce-tp4028454p4028474.html >>> To unsubscribe from Comparing Riak MapReduce and Hadoop MapReduce, click >>> here. >>> NAML<http://riak-users.197444.n3.nabble.com/template/NamlServlet.jtp?macro=macro_viewer&id=instant_html%21nabble%3Aemail.naml&base=nabble.naml.namespaces.BasicNamespace-nabble.view.web.template.NabbleNamespace-nabble.naml.namespaces.BasicNamespace-nabble.view.web.template.NabbleNamespace-nabble.view.web.template.NodeNamespace&breadcrumbs=notify_subscribers%21nabble%3Aemail.naml-instant_emails%21nabble%3Aemail.naml-send_instant_email%21nabble%3Aemail.naml> >>> >> >> >> ------------------------------ >> View this message in context: Re: Comparing Riak MapReduce and Hadoop >> MapReduce<http://riak-users.197444.n3.nabble.com/Comparing-Riak-MapReduce-and-Hadoop-MapReduce-tp4028454p4028476.html> >> >> Sent from the Riak Users mailing list >> archive<http://riak-users.197444.n3.nabble.com/>at >> Nabble.com. >> >> _______________________________________________ >> riak-users mailing list >> [hidden email] <http://user/SendEmail.jtp?type=node&node=4028477&i=1> >> http://lists.basho.com/mailman/listinfo/riak-users_lists.basho.com >> >> >> _______________________________________________ >> riak-users mailing list >> [hidden email] <http://user/SendEmail.jtp?type=node&node=4028477&i=2> >> http://lists.basho.com/mailman/listinfo/riak-users_lists.basho.com >> >> >> ------------------------------ >> If you reply to this email, your message will be added to the >> discussion below: >> >> http://riak-users.197444.n3.nabble.com/Comparing-Riak-MapReduce-and-Hadoop-MapReduce-tp4028454p4028477.html >> To unsubscribe from Comparing Riak MapReduce and Hadoop MapReduce, click >> here. >> NAML<http://riak-users.197444.n3.nabble.com/template/NamlServlet.jtp?macro=macro_viewer&id=instant_html%21nabble%3Aemail.naml&base=nabble.naml.namespaces.BasicNamespace-nabble.view.web.template.NabbleNamespace-nabble.naml.namespaces.BasicNamespace-nabble.view.web.template.NabbleNamespace-nabble.view.web.template.NodeNamespace&breadcrumbs=notify_subscribers%21nabble%3Aemail.naml-instant_emails%21nabble%3Aemail.naml-send_instant_email%21nabble%3Aemail.naml> >> > > > ------------------------------ > View this message in context: Re: Comparing Riak MapReduce and Hadoop > MapReduce<http://riak-users.197444.n3.nabble.com/Comparing-Riak-MapReduce-and-Hadoop-MapReduce-tp4028454p4028486.html> > Sent from the Riak Users mailing list > archive<http://riak-users.197444.n3.nabble.com/>at Nabble.com. > > _______________________________________________ > riak-users mailing list > riak-users@lists.basho.com > http://lists.basho.com/mailman/listinfo/riak-users_lists.basho.com > >
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