go/src/runtime/mcentral.go:19
type mcentral struct {
	lock      mutex
	spanclass spanClass
	nonempty  mSpanList // list of spans with a free object, ie a nonempty free list
	empty     mSpanList // list of spans with no free objects (or cached in an mcache)

	// nmalloc is the cumulative count of objects allocated from
	// this mcentral, assuming all spans in mcaches are
	// fully-allocated. Written atomically, read under STW.
	nmalloc uint64
}

go/src/runtime/mgcsweep.go:364
func (s *mspan) sweep(preserve bool) bool {
    ...
    wasempty := s.nextFreeIndex() == s.nelems
    nalloc := uint16(s.countAlloc())
    nfreed := s.allocCount - nalloc
    ...
    if nfreed > 0 && spc.sizeclass() != 0 {
		c.local_nsmallfree[spc.sizeclass()] += uintptr(nfreed)
		res = mheap_.central[spc].mcentral.freeSpan(s, preserve, wasempty)
		// mcentral.freeSpan updates sweepgen
    } 
    ...
}

go/src/runtime/mcentral.go:228
func (c *mcentral) freeSpan(s *mspan, preserve bool, wasempty bool) bool {
	...

	// Move to nonempty if necessary.
	if wasempty {
		c.empty.remove(s)
		c.nonempty.insert(s)
	}

	...
}

go/src/runtime/mcentral.go:228
func (c *mcentral) freeSpan(s *mspan, preserve bool, wasempty bool) bool {
	...

	// Move to nonempty if necessary.
	if wasempty {
		c.nonempty.remove(s)
                 c.empty.insert(s)
	}

	...
}