What version of Go are you using (go version)?

go1.21.2

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

N/A

What did you do?

Import expvar, consume all registered keys.

What did you expect to see?

No application-wide impact.

What did you see instead?

The "memstats" key calls the old runtime.ReadMemStats API, which involves an application-wide stop-the-world while it collects the most up-to-date possible data.

https://cs.opensource.google/go/go/+/refs/tags/go1.21.2:src/expvar/expvar.go;l=365

The expvar package doesn't guarantee the most up-to-date data. Users who need that (test code that wants to measure single allocations) would use runtime.ReadMemStats directly. We should be able to get a version of the data that's sufficient for expvar's use without the app-wide STW; most of it is available in runtime/metrics already.

I intend to tackle this over the next several weeks.

CC @golang/runtime , and @mknyszek in particular