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https://issues.apache.org/jira/browse/CRUNCH-602?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15250151#comment-15250151
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Michael Rose commented on CRUNCH-602:
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At least in our environment that seems to be the case. Our NN was on really
weak hardware and it consistently hit 100% CPU while jobs with combiners were
running, which could just be basic metadata RPC traffic. I'm not sure if it was
grabbing the contents each time. In any case, it would be better to avoid the
M*R network calls during map phase.
I think I'll have time today to reduce debug DistCache call down a bit and see
which RPCs the filesystem calls are sending and if it's possible to cache
those, given that the distributed cache shouldn't be changing mid-job.
> 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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