func extrapolatedRate(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper, isCounter bool, isRate bool) Vector {
	ms := args[0].(*parser.MatrixSelector)
	vs := ms.VectorSelector.(*parser.VectorSelector)
	var (
	    // samples表示某个metric的某段时间的数据（区间向量）
		samples    = vals[0].(Matrix)[0]
		
		// enh.Ts: 执行查询的时间.
		// ms.Range: 区间向量表达式中括号内的时间转换成Duration.
		// vs.Offset: 区间向量表达式中后面跟的offset 的时间转换成Duration.
		rangeStart = enh.Ts - durationMilliseconds(ms.Range+vs.Offset)
		rangeEnd   = enh.Ts - durationMilliseconds(vs.Offset)
	)

	// No sense in trying to compute a rate without at least two points. Drop
	// this Vector element.
	if len(samples.Points) < 2 {
		return enh.Out
	}

	resultValue := samples.Points[len(samples.Points)-1].V - samples.Points[0].V
	// 如果exporter被重启，conter会从头开始计数，跟前面就不对应了，下面是斧正的逻辑
	if isCounter {
		var lastValue float64
		for _, sample := range samples.Points {
			if sample.V < lastValue {
				resultValue += lastValue
			}
			lastValue = sample.V
		}
	}

	// Duration between first/last samples and boundary of range.
	// 区间向量第一个指标的时间戳 - rangeStart，除1000表示以Milliseconds为时间单位。
	durationToStart := float64(samples.Points[0].T-rangeStart) / 1000
    // 区间向量最后个指标（距当前时间最近一次的指标）的时间戳 - rangeStart，除1000表示以Milliseconds为时间单位。
	durationToEnd := float64(rangeEnd-samples.Points[len(samples.Points)-1].T) / 1000

    // 取区间向量第一个和最后一个指标时间的差值，除1000表示以Milliseconds为时间单位。
	sampledInterval := float64(samples.Points[len(samples.Points)-1].T-samples.Points[0].T) / 1000
	// 平均时间间隔
	averageDurationBetweenSamples := sampledInterval / float64(len(samples.Points)-1)

	if isCounter && resultValue > 0 && samples.Points[0].V >= 0 {
		// Counters cannot be negative. If we have any slope at
		// all (i.e. resultValue went up), we can extrapolate
		// the zero point of the counter. If the duration to the
		// zero point is shorter than the durationToStart, we
		// take the zero point as the start of the series,
		// thereby avoiding extrapolation to negative counter
		// values.
		durationToZero := sampledInterval * (samples.Points[0].V / resultValue)
		if durationToZero < durationToStart {
			durationToStart = durationToZero
		}
	}

	// If the first/last samples are close to the boundaries of the range,
	// extrapolate the result. This is as we expect that another sample
	// will exist given the spacing between samples we've seen thus far,
	// with an allowance for noise.
	extrapolationThreshold := averageDurationBetweenSamples * 1.1
	extrapolateToInterval := sampledInterval

	if durationToStart < extrapolationThreshold {
		extrapolateToInterval += durationToStart
	} else {
		extrapolateToInterval += averageDurationBetweenSamples / 2
	}
	if durationToEnd < extrapolationThreshold {
		extrapolateToInterval += durationToEnd
	} else {
		extrapolateToInterval += averageDurationBetweenSamples / 2
	}
	resultValue = resultValue * (extrapolateToInterval / sampledInterval)
	if isRate {
		resultValue = resultValue / ms.Range.Seconds()
	}

	return append(enh.Out, Sample{
		Point: Point{V: resultValue},
	})
}


// === rate(node parser.ValueTypeMatrix) Vector ===
func funcRate(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper) Vector {
	return extrapolatedRate(vals, args, enh, true, true)
}

// === rate(node parser.ValueTypeMatrix) Vector ===
func funcRate(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper) Vector {
	return extrapolatedRate(vals, args, enh, true, true)
}

// === irate(node parser.ValueTypeMatrix) Vector ===
func funcIrate(vals []parser.Value, args parser.Expressions, enh *EvalNodeHelper) Vector {
	return instantValue(vals, enh.Out, true)
}
....

func instantValue(vals []parser.Value, out Vector, isRate bool) Vector {
	samples := vals[0].(Matrix)[0]
	// No sense in trying to compute a rate without at least two points. Drop
	// this Vector element.
	if len(samples.Points) < 2 {
		return out
	}

	lastSample := samples.Points[len(samples.Points)-1]
	previousSample := samples.Points[len(samples.Points)-2]

	var resultValue float64
	if isRate && lastSample.V < previousSample.V {
		// Counter reset.
		resultValue = lastSample.V
	} else {
		resultValue = lastSample.V - previousSample.V
	}

	sampledInterval := lastSample.T - previousSample.T
	if sampledInterval == 0 {
		// Avoid dividing by 0.
		return out
	}

	if isRate {
		// Convert to per-second.
		resultValue /= float64(sampledInterval) / 1000
	}

	return append(out, Sample{
		Point: Point{V: resultValue},
	})
}