congestion: don't use floating point math when calculating pacing times

internal/congestion/pacer.go

@@ -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) {

internal/congestion/pacer_test.go

@@ -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)