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From "Micah Whitacre (JIRA)" <>
Subject [jira] [Commented] (CRUNCH-602) Combiner initialization repeatedly retrieves RT nodes from DistCache, leading to high NN load
Date Fri, 02 Sep 2016 20:38:20 GMT


Micah Whitacre commented on CRUNCH-602:

I didn't have a direct suggestion was mostly just trying to see if this was a still an issue
we should look at integrating into the next release or if it had been found to not be an issue.

Out of curiosity how many files are you putting into your distcache?  Also what is the normal
size of those files?  Since it only caches the 10MB or less files, I'm wondering if it will
cause problems for others who might have larger files on their cache or more/less than you.

> Combiner initialization repeatedly retrieves RT nodes from DistCache, leading to high
NN load
> ---------------------------------------------------------------------------------------------
>                 Key: CRUNCH-602
>                 URL:
>             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>), 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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