Do unused actors hold thread/resources?

If I create a bunch of actors and don’t assign any work yet to them, do they hold thread/cpu resources or are they passive until the first message in the mailbox is processed?

They don’t hold thread/cpu resources.

Actors share a thread pool (dispatcher) and are only active when messages are scheduled to them.
You can pin an actor to a dedicated thread, but that is uncommon usage and not the default.

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