When we introduced the "compare" step in the performance projects in Evergreen, the ratio calculated was put in perf.json file in the /src directory of the machines running that script. We then waited for the post-step in the yml file to send that result to Evergreen.
This causes an annoying error message to show up for the non-compare tasks. Here is an example of that message from a mongo-perf test.
In PERF-288, I am making the compare.py script to only put out the ratios and not making decision on pass/fail. We can put the json.send in the "compare" tasks now instead of in the post-step.