cmd/go: go list does not scale with GOMAXPROCS

[goto1134]

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

master ace1494

Does this issue reproduce with the latest release?

yes

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

go env Output

$ go env
GOARCH='arm64'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOOS='darwin'

What did you do?

1. Clone Kubernetes
2. Run GOMAXPROCS=10 go list -json -m -u -mod=readonly -debug-trace=trace-default.txt all
3. Run GOMAXPROCS=20 go list -json -m -u -mod=readonly -debug-trace=trace-maxproc.txt all

The second command takes the same amount of time as the first one.
Here are the collected traces:

What did you expect to see?

The second command runs faster and scales, depending on the GOMAXPROCS value.

What did you see instead?

The second comment took the same amount of time as the first one.

Here are the collected traces:
trace-default.txt
trace-maxproc20.txt

From the traces, it's evident that the bottleneck is in getting the modules in a loop here:
https://github.com/golang/go/blob/ace1494d9235be94f1325ab6e45105a446b3224c/src/cmd/go/internal/modload/list.go#L277C1-L286C4

[gopherbot]

Change https://go.dev/cl/530037 mentions this issue: cmd/go: scale go list with GOMAXPROCS

[dany74q]

Any update on this one ? It makes life much better for large multi-module mono-repos

[goto1134]

@dany74q , the next step is to provide benchmarks. I want to do it sometime later. Probably in November.

I hope this change will be accepted for the February release of Go.