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.