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congestion: don't use floating point math when calculating pacing times (#4148)

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Marten Seemann 2023-10-31 16:21:42 +07:00 • committed by GitHub
parent a3603549ee
commit f23da7da47
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2 changed files with 23 additions and 7 deletions
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19
internal/congestion/pacer.go
View file

@ -1,7 +1,6 @@
package congestion
import (
"math"
"time"
"github.com/quic-go/quic-go/internal/protocol"
@ -26,7 +25,7 @@ func newPacer(getBandwidth func() Bandwidth) *pacer {
bw := uint64(getBandwidth() / BytesPerSecond)
// Use a slightly higher value than the actual measured bandwidth.
// RTT variations then won't result in under-utilization of the congestion window.
// Ultimately, this will result in sending packets as acknowledgments are received rather than when timers fire,
// Ultimately, this will result in sending packets as acknowledgments are received rather than when timers fire,
// provided the congestion window is fully utilized and acknowledgments arrive at regular intervals.
return bw * 5 / 4
},
@ -37,7 +36,7 @@ func newPacer(getBandwidth func() Bandwidth) *pacer {
func (p *pacer) SentPacket(sendTime time.Time, size protocol.ByteCount) {
budget := p.Budget(sendTime)
if size > budget {
if size >= budget {
p.budgetAtLastSent = 0
} else {
p.budgetAtLastSent = budget - size
@ -69,10 +68,16 @@ func (p *pacer) TimeUntilSend() time.Time {
if p.budgetAtLastSent >= p.maxDatagramSize {
return time.Time{}
}
return p.lastSentTime.Add(utils.Max(
protocol.MinPacingDelay,
time.Duration(math.Ceil(float64(p.maxDatagramSize-p.budgetAtLastSent)*1e9/float64(p.adjustedBandwidth())))*time.Nanosecond,
))
diff := 1e9 * uint64(p.maxDatagramSize-p.budgetAtLastSent)
bw := p.adjustedBandwidth()
// We might need to round up this value.
// Otherwise, we might have a budget (slightly) smaller than the datagram size when the timer expires.
d := diff / bw
// this is effectively a math.Ceil, but using only integer math
if diff%bw > 0 {
d++
}
return p.lastSentTime.Add(utils.Max(protocol.MinPacingDelay, time.Duration(d)*time.Nanosecond))
}
func (p *pacer) SetMaxDatagramSize(s protocol.ByteCount) {

11
internal/congestion/pacer_test.go
View file

@ -101,6 +101,17 @@ var _ = Describe("Pacer", func() {
Expect(p.Budget(t.Add(5 * t2.Sub(t)))).To(BeEquivalentTo(5 * packetSize))
})
It("has enough budget for at least one packet when the timer expires", func() {
t := time.Now()
sendBurst(t)
for bw := uint64(100); bw < uint64(5*initialMaxDatagramSize); bw++ {
bandwidth = bw // reduce the bandwidth to 5 packet per second
t2 := p.TimeUntilSend()
Expect(t2).To(BeTemporally(">", t))
Expect(p.Budget(t2)).To(BeNumerically(">=", initialMaxDatagramSize))
}
})
It("never allows bursts larger than the maximum burst size", func() {
t := time.Now()
sendBurst(t)