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// 如果 stageOne 没满，整个 LFU 区域也没满
	if slru.stageOne.Len() < slru.stageOneCap || slru.Len() < slru.stageOneCap+slru.stageTwoCap {
		slru.data[newitem.key] = slru.stageOne.PushFront(&newitem)
		return
	}

// 并行的运行子压缩情况
func (lm *levelManager) addSplits(cd *compactDef) {
	cd.splits = cd.splits[:0]
	// Let's say we have 10 tables in cd.bot and min width = 3. Then, we'll pick
	// 0, 1, 2 (pick), 3, 4, 5 (pick), 6, 7, 8 (pick), 9 (pick, because last table).
	// This gives us 4 picks for 10 tables.
	// In an edge case, 142 tables in bottom led to 48 splits. That's too many splits, because it
	// then uses up a lot of memory for table builder.
	// We should keep it so we have at max 5 splits.
	width := int(math.Ceil(float64(len(cd.bot)) / 5.0))
	if width < 3 {
		width = 3
	}
	skr := cd.thisRange
	skr.extend(cd.nextRange)
	addRange := func(right []byte) {
		skr.right = utils.Copy(right)
		cd.splits = append(cd.splits, skr)
		skr.left = skr.right
	}
	for i, t := range cd.bot {
		// last entry in bottom table.
		// 最后一个range分配无穷大的空间
		if i == len(cd.bot)-1 {
			addRange([]byte{})
			return
		}
		// 这里实现的是每(cd.bot/5)个合并成一个
		if i%width == width-1 {
			// 设置最大值为右区间
			right := utils.KeyWithTs(utils.ParseKey(t.ss.MaxKey()), math.MaxUint64)
			addRange(right)
		}
	}
}