// must ensure that allocation cannot happen around the
	// access (since that may free the backing store).
	allspans []*mspan // all spans out there

	// Proportional sweep
	//
	// These parameters represent a linear function from gcController.heapLive
	// to page sweep count. The proportional sweep system works to
	// stay in the black by keeping the current page sweep count
	// above this line at the current gcController.heapLive.
	//