### Jira - [x] My PR addresses the following [Airflow Jira](https://issues.apache.org/jira/browse/AIRFLOW-2156) issues and references them in the PR title. - https://issues.apache.org/jira/browse/AIRFLOW-2156
### Description This change is mostly authored by @aoen. I am merely doing the tests/small fix and PR publishing due to job change of him. The change would bring Airflow to meet 5 min scheduling SLA with 30k running tasks according to Airbnb production traffic and stress tests. The performance of the celery querying step is hugely improved by 15x+ with 16 processors and can potentially be fast with more processors. - [x] Here are some details about my PR, including screenshots of any UI changes:  <img width="1586" alt="screen shot 2018-06-15 at 12 40 18 am copy" src="https://user-images.githubusercontent.com/7818710/44940495-36964800-ad44-11e8-9a4a-3ff7790d14a9.png";> Notes: Syncing no longer happens at the end of the celery executor executions (e.g. if scheduler shuts down). The sync does not actually guarantee that tasks finished anyways and prolongs the ending protocol. There is no timeout on the subprocesses but that wasn't the case before this change either What about logging for the multiprocessing tasks? Well it's ok to skip them, they aren't currently logged either. ### Tests - [x] My PR adds the following unit tests: tests/executors/test_celery_executor.py:CeleryExecutorTest.test_exception_propagation ### Commits - [x] My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "[How to write a good git commit message](http://chris.beams.io/posts/git-commit/)": 1. Subject is separated from body by a blank line 1. Subject is limited to 50 characters (not including Jira issue reference) 1. Subject does not end with a period 1. Subject uses the imperative mood ("add", not "adding") 1. Body wraps at 72 characters 1. Body explains "what" and "why", not "how" ### Documentation - [ ] In case of new functionality, my PR adds documentation that describes how to use it. - When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added. ### Code Quality - [x] Passes `git diff upstream/master -u -- "*.py" | flake8 --diff` [ Full content available at: https://github.com/apache/incubator-airflow/pull/3830 ] This message was relayed via gitbox.apache.org for [email protected]
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