HFileUtils when preparing for writing HFiles sets the number of reducers equal to the number of regions in the table, and then writes out the start keys for each region to a sequence file for the TotalOrderPartitioner to consume when partitioning data. This can result in a very large quantity of reducers that don't do anything due to not having any data to write to hfiles for the region its partition belonged to.
My proposal is to modify HFileUtils, with an optional parameter (or a config, that's up for debate) to determine which regions data will be loaded into ahead of time, and set the number of reducers to equal the number of regions, and only write out the start keys for those affected regions.
I have working code to do this on the 0.8.x branch of crunch, as that is what I am currently on. I can modify it to work on more recent versions, but I wanted to start a discussion around the viability of this code being contributed back to the community. I am still in process of capturing metrics around the impact of the change (and trying to get data large enough to test this out), but at least from a reducer count I have seen substantial drops in my limited testing so far. For example, I had a job go from 705 reduce tasks during the write down to 36 reduce tasks.
I've attached what I have so far as of 0.8.4. I'm going to start working on a version modified for the latest version of crunch.