Why github actions start so fast?

hi. i found github actions start job very very fast.
does github actions use kubernetes for run job? or vm ?

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The GitHub hosted runners are VMs. The publicly available details on them are here: About GitHub-hosted runners - GitHub Docs

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The VMs are generally already waiting for work, so you aren’t generally waiting for a VM to boot.

You can imagine that there’s a pool of idle VMs running the action host and registered to GitHub to receive work. When GitHub encounters an event that would trigger an action, it packages up the event, picks an eligible runner and dispatches the task to it.

It’s likely that GitHub scales up/down runners as its pool of available runners decreases/increases due to work being assigned to runners/completed.

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thank your answer. it is very useful to me~