Celery: number of workers vs concurrency

What is the difference between having:

1. one worker with concurrency 4 or
2. two workers with concurrency 2 each

for the same queue.

Thanks

Intersting question.

Things that I can think of (I'm sure there are a lot more):

• For having high availability:
  1. You want more than one machine (if one goes down) - so you must use worker per machine.
  2. Even for one machine - I think it is safer to have 2 workers which run in a two different processes instead of one worker with high concurrency (correct me if I wrong, but I think it is implemented with threads).
• In docs I see that they the recommendation is to use concurrency per CPUs.
• If you want to separate different tasks to different workers..

Of course, you have price for that: more processes that takes more resources (CPU/Memory etc).

I found this question which is quite similar.