[GitHub] [airflow] ashb commented on issue #26933: Low priority tasks are scheduled before high priority tasks

[GitBox <gi...@apache.org>]

ashb commented on issue #26933:
URL: https://github.com/apache/airflow/issues/26933#issuecomment-1375297905

   This is an "unavoidable" side-effect of wanting high throughput from the scheduler. If you want the scheduler to schedule tasks as fast as possible, then task priority will only come in to effect when there are more scheduled tasks waiting than the queue slots.
   
   If you've ever tried to implement QoS of a broadband connection to have SSH/interactive sessions not get throttled by large downloads then the same thing applies there: if you allow 100% use of the bandwidth then there is no possibility for higher priority traffic to be placed ahead of bulk traffic. But if you only allow bulk traffic to take 90% of the pipe then you have some slack to put higher priorty things ahead.
   
   There isn't quite a direct analogy in terms of the scheduler, but something might be achievable right now with using Queues. And it's _possible_ the scheduler could be extended to do some kind of "look ahead" to see what high priority tasks might be coming up soon, but that is a decidedly non-trivial amount of work.
   
   So I think I'm tempted to close this as "Sorry, can't fix"/docs need expanding to clarify this behaviour.


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