func (p *pageAlloc) scavenge(nbytes uintptr, shouldStop func() bool, force bool) uintptr {
	released := uintptr(0)
	for released < nbytes {
		ci, pageIdx := p.scav.index.find(force)
		if ci == 0 {
			break
		}
		systemstack(func() {
			released += p.scavengeOne(ci, pageIdx, nbytes-released)
		})
		if shouldStop != nil && shouldStop() {
			break
		}
	}
	return released
}

	Sleep      func(int64) int64
	Scavenge   func(uintptr) (uintptr, int64)
	ShouldStop func() bool
	GoMaxProcs func() int32

	released  atomic.Uintptr
	scavenger scavengerState
	stop      chan<- struct{}
	done      <-chan struct{}
}

func (s *Scavenger) Start() {
	if s.Sleep == nil || s.Scavenge == nil || s.ShouldStop == nil || s.GoMaxProcs == nil {
		panic("must populate all stubs")
	}