Analysis of doOnSubscribe behavior in RxJava chain

In the updateRegions method, getRegions is called, which returns a Single with a list of regions, and the thread does not switch inside the chain. It is necessary to explain on which Scheduler the code passed to doOnSubscribe will be executed.

fun updateRegions() {
    getRegions()
        .subscribeOn(Schedulers.single())
        .observeOn(Schedulers.computation())
        .doOnSubscribe {
            showProgress()
        }
        .subscribeOn(Schedulers.io())
        .subscribe(
            { regions ->
                showRegions(regions)
            }
        )
}

Describe which thread will be used to call showProgress() and why.