pool/adaptive.go

package pool

import (
	"code.squareroundforest.org/arpio/times"
	"math"
	"time"
)

const (
	// arbitrary values to be most likely out of sync with anything else:
	minNightshiftTime = 729 * time.Millisecond // ~1sec
	maxNightshiftTime = 59049 * time.Second    // ~2/3day
)

type adaptive struct {
	clock       times.Clock
	prevState   Stats
	initialized bool
	activeStart time.Time
	activeEnd   time.Time
	average     float64
	deviation   float64
}

func makeAdaptiveAlgo() *adaptive {
	return &adaptive{}
}

func (a *adaptive) setClock(c times.Clock) {
	a.clock = c
}

func movingAverage(prev, currv float64) float64 {
	return prev + (currv-prev)/math.E
}

func movingAbsoluteDeviation(prev, currv, currav float64) float64 {
	return prev + (math.Abs(currv-currav)-prev)/math.E
}

func targetCapacity(av, dev float64) float64 {
	return av + dev*math.E
}

func (a *adaptive) target(s Stats) int {
	av := movingAverage(a.average, float64(s.Active))
	dev := movingAbsoluteDeviation(a.deviation, float64(s.Active), av)
	a.average = av
	a.deviation = dev
	return int(targetCapacity(av, dev))
}

func (a *adaptive) nightshiftTO() time.Duration {
	to := a.activeEnd.Sub(a.activeStart)
	to = (3 * to) / 8
	if to < minNightshiftTime {
		to = minNightshiftTime
	}

	if to > maxNightshiftTime {
		to = maxNightshiftTime
	}

	return to
}

func (a *adaptive) nightshift(s Stats) time.Duration {
	pactive := a.prevState.Active > 0
	empty := s.Idle == 0
	active := s.Active > 0
	a.prevState = s

	// on the first put we need to set an active start time:
	if !a.initialized {
		a.initialized = true
		a.activeStart = a.clock.Now()
		if active {
			return 0
		}

		return a.nightshiftTO()
	}

	if !pactive && !active && !empty {
		return a.nightshiftTO()
	}

	if !pactive && active {
		a.activeStart = a.clock.Now()
		return 0
	}

	if pactive && !active {
		a.activeEnd = a.clock.Now()
		if empty {
			return 0
		}

		return a.nightshiftTO()
	}

	return 0
}

func (a *adaptive) Target(s Stats) (int, time.Duration) {
	t := a.target(s)

	// magic number 2: we allow max 2 idle items to be collected only by the nightshift, to provide better
	// support for sporadic requests, when it's active or just going inactive:
	if t < 2 && a.activeStart.After(a.activeEnd) {
		t = 2
	}

	ns := a.nightshift(s)
	return t, ns
}

func (a *adaptive) Load(n int) {
	// we lie to the algorithm when adding the additional idle count to the average active count. This way
	// we can adjust the calculated target capacity:
	a.average += float64(n)
	s := a.prevState
	s.Idle += n
	a.nightshift(s)
}