A simple counting step following a group by key is extremely slow in a DataFlow pipeline
问题 I have a DataFlow pipeline trying to build an index (key-value pairs) and compute some metrics (like a number of values per key). The input data is about 60 GB total, stored on GCS and the pipeline has about 126 workers allocated. Per Stackdriver all workers have about 6% CPU utilization. The pipeline seems to make no progress despite having 126 workers and based on the wall time the bottleneck seems to be a simple counting step that follows a group by. While all other steps have on average