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From "Dmitry Goldin (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CRUNCH-602) Combiner initialization repeatedly retrieves RT nodes from DistCache, leading to high NN load
Date Fri, 14 Oct 2016 16:11:20 GMT

    [ https://issues.apache.org/jira/browse/CRUNCH-602?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15575743#comment-15575743
] 

Dmitry Goldin commented on CRUNCH-602:
--------------------------------------

Hi,

We have also discovered this issue on our cluster and the issue causes severe load on the
Namenode, especially since we have fairly large crunch jobs (tens of thousands of mappers
and hundreds or thousands of reducers). 

The amount of reads to the COMBINE file is orders of magnitude larger than necessary. I.e,
instead of roughly $num_mappers*$num_spills + $num_reducers we have roughly $num_mappers*$num_spills*$num_reducers
(simplified)

The initial description of this issue is pretty spot on, but there are additional scenarios
when combiners are initialised, such as for each spill and it extends to MAP and REDUCE files
too, but the effect there does not amplify so strongly. If a more detailed analysis is required,
I can try to add some of the internal investigation I have been doing.

At the moment we are testing a patched version internally which alleviates the issue as far
as I can judge.
I will attach the patch for information purposes.

> Combiner initialization repeatedly retrieves RT nodes from DistCache, leading to high
NN load
> ---------------------------------------------------------------------------------------------
>
>                 Key: CRUNCH-602
>                 URL: https://issues.apache.org/jira/browse/CRUNCH-602
>             Project: Crunch
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 0.12.0, 0.13.0, 0.14.0
>         Environment: Crunch 0.14-SNAPSHOT, CDH5.6.0
>            Reporter: Michael Rose
>            Assignee: Josh Wills
>              Labels: performance
>         Attachments: crunch-602.patch
>
>
> When running one of our Crunch pipelines, we noticed our NameNode under very heavy load.
We run our masters on pretty light hardware, so our NN was sitting at 100% CPU.
> Crunch reads the RTNodes during creation of a CrunchTaskContext. These are created when
Mappers and Reducers are created. Importantly, a CrunchCombiner is a subclass of a Reducer,
so each mapper will create R combiners where R is the number of reducers and thus R CrunchTaskContexts.
Consequently in highly parallel jobs, this means M*R semi-expensive calls to the NameNode.
> In the constructor for CrunchTaskContext, this is the read to the DistCache:
> this.nodes = (List<RTNode>) DistCache.read(conf, path);
> Which then leads to a read into the NN + deserialization.
> For now, we took the overly simplistic approach of caching the results of the DistCache
read in a Guava cache. The cache ensures combiners reuse RTNodes with only the overhead of
deserialization which is somewhat unavoidable as RTNodes are stateful and not reusable. However,
it's not configurable except by modifying code.
> I'll attach the patch, but given that it's not yet configurable I wouldn't call it a
"fix available." There may be much better ways of fixing this issue as well -- if you have
some guidance I'd be happy to do the legwork on a patch.



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