From 2408301c987b832c4a8f0ce2ceda140043489a30 Mon Sep 17 00:00:00 2001
From: Toby <tobyxdd@gmail.com>
Date: Sun, 14 Apr 2024 15:07:43 -0700
Subject: [PATCH] feat: pacer code improvements

---
 core/internal/congestion/common/pacer.go | 46 ++++++++----------------
 1 file changed, 15 insertions(+), 31 deletions(-)

diff --git a/core/internal/congestion/common/pacer.go b/core/internal/congestion/common/pacer.go
index 4e089a3147..6adbd813c2 100644
--- a/core/internal/congestion/common/pacer.go
+++ b/core/internal/congestion/common/pacer.go
@@ -1,14 +1,14 @@
 package common
 
 import (
-	"math"
 	"time"
 
 	"github.com/apernet/quic-go/congestion"
 )
 
 const (
-	maxBurstPackets = 10
+	maxBurstPackets               = 10
+	maxBurstPacingDelayMultiplier = 4
 )
 
 // Pacer implements a token bucket pacing algorithm.
@@ -46,12 +46,12 @@ func (p *Pacer) Budget(now time.Time) congestion.ByteCount {
 	if budget < 0 { // protect against overflows
 		budget = congestion.ByteCount(1<<62 - 1)
 	}
-	return minByteCount(p.maxBurstSize(), budget)
+	return min(p.maxBurstSize(), budget)
 }
 
 func (p *Pacer) maxBurstSize() congestion.ByteCount {
-	return maxByteCount(
-		congestion.ByteCount((congestion.MinPacingDelay+time.Millisecond).Nanoseconds())*p.getBandwidth()/1e9,
+	return max(
+		congestion.ByteCount((maxBurstPacingDelayMultiplier*congestion.MinPacingDelay).Nanoseconds())*p.getBandwidth()/1e9,
 		maxBurstPackets*p.maxDatagramSize,
 	)
 }
@@ -62,34 +62,18 @@ func (p *Pacer) TimeUntilSend() time.Time {
 	if p.budgetAtLastSent >= p.maxDatagramSize {
 		return time.Time{}
 	}
-	return p.lastSentTime.Add(maxDuration(
-		congestion.MinPacingDelay,
-		time.Duration(math.Ceil(float64(p.maxDatagramSize-p.budgetAtLastSent)*1e9/
-			float64(p.getBandwidth())))*time.Nanosecond,
-	))
+	diff := 1e9 * uint64(p.maxDatagramSize-p.budgetAtLastSent)
+	bw := uint64(p.getBandwidth())
+	// 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(max(congestion.MinPacingDelay, time.Duration(d)*time.Nanosecond))
 }
 
 func (p *Pacer) SetMaxDatagramSize(s congestion.ByteCount) {
 	p.maxDatagramSize = s
 }
-
-func maxByteCount(a, b congestion.ByteCount) congestion.ByteCount {
-	if a < b {
-		return b
-	}
-	return a
-}
-
-func minByteCount(a, b congestion.ByteCount) congestion.ByteCount {
-	if a < b {
-		return a
-	}
-	return b
-}
-
-func maxDuration(a, b time.Duration) time.Duration {
-	if a > b {
-		return a
-	}
-	return b
-}