Coalesce is reducing the parallelization of your last stage, in your case to 1 task. So, it’s natural it will give poor performance especially with large data. If you absolutely need a single file output, you can instead add a stage boundary and use repartition(1). This will give your query full parallelization during processing while at the end giving you a single task that writes data out. Note that if the file is large (e.g. in 1GB or more) you’ll probably still notice slowness while writing. You may want to reconsider the 1-file requirement for larger datasets.
From: James Yu <ja...@ispot.tv> Date: Wednesday, February 3, 2021 at 1:54 PM To: user <user@spark.apache.org> Subject: Poor performance caused by coalesce to 1 Hi Team, We are running into this poor performance issue and seeking your suggestion on how to improve it: We have a particular dataset which we aggregate from other datasets and like to write out to one single file (because it is small enough). We found that after a series of transformations (GROUP BYs, FLATMAPs), we coalesced the final RDD to 1 partition before writing it out, and this coalesce degrade the performance, not that this additional coalesce operation took additional runtime, but it somehow dictates the partitions to use in the upstream transformations. We hope there is a simple and useful way to solve this kind of issue which we believe is quite common for many people. Thanks James
