Comment by jeffbee

These are my personal observations. You can reproduce, but you'll need to bootstrap the Go toolchain first. For example, in your GOROOT:

  go test -test.bench=Cond2 --test.run=ZZ --test.cpu=1,2,4 sync

For me:

  goos: linux
  goarch: amd64
  pkg: sync
  BenchmarkCond2       3575089        336 ns/op
  BenchmarkCond2-2     3236198        370 ns/op
  BenchmarkCond2-4     2831134        420 ns/op

  goos: darwin
  goarch: arm64
  pkg: sync
  BenchmarkCond2       5903233        190.4 ns/op
  BenchmarkCond2-2       85038      27434 ns/op
  BenchmarkCond2-4     2500750        677.9 ns/op

Way, way worse in the 2- and 4-thread cases on the mac. I don't know why, and the performance of the 2- case swings all over the place.