[stewart27]

Cloud Functions PM here.

Your intuition around concurrency is correct: Cloud Functions has "per instance concurrency" of 1. Cloud Run lets you go significantly higher than that (default 80). This means that our infrastructure will generally create more instances to handle a request spike when using Cloud Functions vs. Cloud Run.

Creating an instance incurs a cold start. Part of that cold start is due to our infrastructure (generally this is small) but the other part is in your control. For example: if you create a client that takes X seconds to you initialize, your cold start will be at least X seconds. The initialization time will manifest as part of your cold start.

This has a few practical implications:

* writing code for Cloud Functions is generally more straightforward as single concurrency solves many problems regarding shared variables. You may also see some benefits in terms of monitoring/metrics/logging since you only need to think about one request at a time.

* you will likely see a higher incidence of cold starts on Cloud Functions during rapid scale-up, such as in response to a sudden traffic spike

* the impact of a given cold start will depend heavily on what you're doing in your container

* though I haven't validated this experimentally, I would expect that the magnitude of any given cold start (i.e., total latency contribution) would be roughly the same on Cloud Run as Cloud Functions IF you're running the same code

[dstaley]

Ah, thanks for the details there! So, given that my Cloud Functions project is a Go app (and would be the exact same code between Functions and Run), if I were to run that in a very minimal container (something like Alpine), I could get roughly the same cold start time as Cloud Functions, but fewer of them since I can respond to multiple requests using the same instance.

I'll probably do some experimentation on my end as well to test. Any suggestion how long I should wait between tests to ensure a cold start on both Cloud Functions and Cloud Run?

[stewart27]

I think you can force cold starts between your tests by re-deploying your function/container. You could (optionally) leave a small buffer (<1 minute) after the deployment to ensure that traffic has fully migrated.