status-go/vendor/github.com/libp2p/go-flow-metrics/sweeper.go

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package flow
import (
"math"
"sync"
"sync/atomic"
"time"
)
// IdleRate the rate at which we declare a meter idle (and stop tracking it
// until it's re-registered).
//
// The default ensures that 1 event every ~30s will keep the meter from going
// idle.
var IdleRate = 1e-13
// Alpha for EWMA of 1s
var alpha = 1 - math.Exp(-1.0)
// The global sweeper.
var globalSweeper sweeper
type sweeper struct {
sweepOnce sync.Once
meters []*Meter
mutex sync.RWMutex
registerChannel chan *Meter
}
func (sw *sweeper) start() {
sw.registerChannel = make(chan *Meter, 16)
go sw.run()
}
func (sw *sweeper) run() {
for m := range sw.registerChannel {
sw.register(m)
sw.runActive()
}
}
func (sw *sweeper) register(m *Meter) {
// Add back the snapshot total. If we unregistered this
// one, we set it to zero.
atomic.AddUint64(&m.accumulator, m.snapshot.Total)
sw.meters = append(sw.meters, m)
}
func (sw *sweeper) runActive() {
ticker := time.NewTicker(time.Second)
defer ticker.Stop()
for len(sw.meters) > 0 {
// Scale back allocation.
if len(sw.meters)*2 < cap(sw.meters) {
newMeters := make([]*Meter, len(sw.meters))
copy(newMeters, sw.meters)
sw.meters = newMeters
}
select {
case t := <-ticker.C:
sw.update(t)
case m := <-sw.registerChannel:
sw.register(m)
}
}
sw.meters = nil
// Till next time.
}
func (sw *sweeper) update(t time.Time) {
sw.mutex.Lock()
defer sw.mutex.Unlock()
for i := 0; i < len(sw.meters); i++ {
m := sw.meters[i]
total := atomic.LoadUint64(&m.accumulator)
diff := total - m.snapshot.Total
if m.snapshot.Rate == 0 {
m.snapshot.Rate = float64(diff)
} else {
m.snapshot.Rate += alpha * (float64(diff) - m.snapshot.Rate)
}
m.snapshot.Total = total
// This is equivalent to one zeros, then one, then 30 zeros.
// We'll consider that to be "idle".
if m.snapshot.Rate > IdleRate {
continue
}
// Ok, so we are idle...
// Mark this as idle by zeroing the accumulator.
swappedTotal := atomic.SwapUint64(&m.accumulator, 0)
// So..., are we really idle?
if swappedTotal > total {
// Not so idle...
// Now we need to make sure this gets re-registered.
// First, add back what we removed. If we can do this
// fast enough, we can put it back before anyone
// notices.
currentTotal := atomic.AddUint64(&m.accumulator, swappedTotal)
// Did we make it?
if currentTotal == swappedTotal {
// Yes! Nobody noticed, move along.
continue
}
// No. Someone noticed and will (or has) put back into
// the registration channel.
//
// Remove the snapshot total, it'll get added back on
// registration.
//
// `^uint64(total - 1)` is the two's compliment of
// `total`. It's the "correct" way to subtract
// atomically in go.
atomic.AddUint64(&m.accumulator, ^uint64(m.snapshot.Total-1))
}
// Reset the rate, keep the total.
m.snapshot.Rate = 0
// remove it and repeat `i`
sw.meters[i] = sw.meters[len(sw.meters)-1]
sw.meters[len(sw.meters)-1] = nil
sw.meters = sw.meters[:len(sw.meters)-1]
i--
}
}
func (sw *sweeper) Register(m *Meter) {
sw.sweepOnce.Do(sw.start)
sw.registerChannel <- m
}