kvflowcontrol: performance optimizations in tracker.go

This patch makes 2 main performance optimizations in the tracker
codepath:
1. Replace the trackedList with a struct that points to a list. This
   avoids map assignments each time anything is appended to the list.
2. We use a sync.Pool for tracked items to avoid frequent allocations
   and gc for the objects.

Informs cockroachdb#104154.

Release note: None

pkg/kv/kvserver/kvflowcontrol/kvflowtokentracker/tracker.go

@@ -15,6 +15,7 @@ import (
 	"fmt"
 	"sort"
 	"strings"
+	"sync"
 
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/kvflowcontrol"
 	"github.com/cockroachdb/cockroach/pkg/kv/kvserver/kvflowcontrol/kvflowcontrolpb"
@@ -29,11 +30,7 @@ import (
 // admissionpb.WorkPriority, for replication along an individual
 // kvflowcontrol.Stream.
 type Tracker struct {
-	// TODO(irfansharif,aaditya): Everytime we track something, we incur a map
-	// assignment (shows up in CPU profiles). We could introduce a struct that
-	// internally embeds this list of tracked deductions, and append there
-	// instead. Do this as part of #104154.
-	trackedM map[admissionpb.WorkPriority][]tracked
+	trackedM map[admissionpb.WorkPriority]*trackedList
 
 	// lowerBound tracks on a per-stream basis the log position below which
 	// we ignore token deductions.
@@ -44,6 +41,24 @@ type Tracker struct {
 	knobs *kvflowcontrol.TestingKnobs
 }
 
+var trackedPool sync.Pool = sync.Pool{
+	New: func() interface{} {
+		return new(tracked)
+	},
+}
+
+func newTracked(tokens kvflowcontrol.Tokens, position kvflowcontrolpb.RaftLogPosition) *tracked {
+	t := trackedPool.Get().(*tracked)
+	t.tokens = tokens
+	t.position = position
+	return t
+}
+
+func freeTracked(t *tracked) {
+	*t = tracked{}
+	trackedPool.Put(t)
+}
+
 // tracked represents tracked flow tokens; they're tracked with respect to a
 // raft log position (typically where the proposed command is expected to end
 // up).
@@ -52,6 +67,10 @@ type tracked struct {
 	position kvflowcontrolpb.RaftLogPosition
 }
 
+type trackedList struct {
+	items []*tracked
+}
+
 // New constructs a new Tracker with the given lower bound raft log position
 // (below which we're not allowed to deduct tokens).
 func New(
@@ -63,7 +82,7 @@ func New(
 		knobs = &kvflowcontrol.TestingKnobs{}
 	}
 	return &Tracker{
-		trackedM:   make(map[admissionpb.WorkPriority][]tracked),
+		trackedM:   make(map[admissionpb.WorkPriority]*trackedList),
 		lowerBound: lb,
 		knobs:      knobs,
 		stream:     stream,
@@ -102,8 +121,12 @@ func (dt *Tracker) Track(
 	}
 	dt.lowerBound = pos
 
-	if len(dt.trackedM[pri]) >= 1 {
-		last := dt.trackedM[pri][len(dt.trackedM[pri])-1]
+	if _, ok := dt.trackedM[pri]; !ok {
+		dt.trackedM[pri] = &trackedList{}
+	}
+
+	if len(dt.trackedM[pri].items) >= 1 {
+		last := dt.trackedM[pri].items[len(dt.trackedM[pri].items)-1]
 		if !last.position.Less(pos) {
 			logFn := log.Errorf
 			if buildutil.CrdbTestBuild {
@@ -115,13 +138,7 @@ func (dt *Tracker) Track(
 		}
 	}
 
-	// TODO(irfansharif,aaditya): The tracked instances here make up about ~0.4%
-	// of allocations under kv0/enc=false/nodes=3/cpu=9. Maybe clean it up as
-	// part of #104154, by using a sync.Pool perhaps.
-	dt.trackedM[pri] = append(dt.trackedM[pri], tracked{
-		tokens:   tokens,
-		position: pos,
-	})
+	dt.trackedM[pri].items = append(dt.trackedM[pri].items, newTracked(tokens, pos))
 	if log.V(1) {
 		log.Infof(ctx, "tracking %s flow control tokens for pri=%s stream=%s pos=%s",
 			tokens, pri, dt.stream, pos)
@@ -144,11 +161,11 @@ func (dt *Tracker) Untrack(
 	var untracked int
 	var tokens kvflowcontrol.Tokens
 	for {
-		if untracked == len(dt.trackedM[pri]) {
+		if untracked == len(dt.trackedM[pri].items) {
 			break
 		}
 
-		deduction := dt.trackedM[pri][untracked]
+		deduction := dt.trackedM[pri].items[untracked]
 		if !deduction.position.LessEq(upto) {
 			break
 		}
@@ -161,17 +178,22 @@ func (dt *Tracker) Untrack(
 		tokens += deduction.tokens
 	}
 
-	trackedBefore := len(dt.trackedM[pri])
-	dt.trackedM[pri] = dt.trackedM[pri][untracked:]
+	trackedBefore := len(dt.trackedM[pri].items)
+
+	// Free up untracked items.
+	for i := 0; i < untracked; i++ {
+		freeTracked(dt.trackedM[pri].items[i])
+	}
+	dt.trackedM[pri].items = dt.trackedM[pri].items[untracked:]
 	if log.V(1) {
 		remaining := ""
-		if len(dt.trackedM[pri]) > 0 {
-			remaining = fmt.Sprintf(" (%s, ...)", dt.trackedM[pri][0].tokens)
+		if len(dt.trackedM[pri].items) > 0 {
+			remaining = fmt.Sprintf(" (%s, ...)", dt.trackedM[pri].items[0].tokens)
 		}
 		log.Infof(ctx, "released %s flow control tokens for %d out of %d tracked deductions for pri=%s stream=%s, up to %s; %d tracked deduction(s) remain%s",
-			tokens, untracked, trackedBefore, pri, dt.stream, upto, len(dt.trackedM[pri]), remaining)
+			tokens, untracked, trackedBefore, pri, dt.stream, upto, len(dt.trackedM[pri].items), remaining)
 	}
-	if len(dt.trackedM[pri]) == 0 {
+	if len(dt.trackedM[pri].items) == 0 {
 		delete(dt.trackedM, pri)
 	}
 
@@ -186,7 +208,7 @@ func (dt *Tracker) Untrack(
 func (dt *Tracker) Iter(_ context.Context, f func(admissionpb.WorkPriority, kvflowcontrol.Tokens)) {
 	for pri, deductions := range dt.trackedM {
 		var tokens kvflowcontrol.Tokens
-		for _, deduction := range deductions {
+		for _, deduction := range deductions.items {
 			tokens += deduction.tokens
 		}
 		f(pri, tokens)
@@ -229,7 +251,7 @@ func (dt *Tracker) TestingIter(
 	f func(admissionpb.WorkPriority, kvflowcontrol.Tokens, kvflowcontrolpb.RaftLogPosition) bool,
 ) {
 	for pri, deductions := range dt.trackedM {
-		for _, deduction := range deductions {
+		for _, deduction := range deductions.items {
 			if !f(pri, deduction.tokens, deduction.position) {
 				return
 			}