From 9eaa74b26dea8e68cd648d5edc570e3257451da0 Mon Sep 17 00:00:00 2001
From: Yiling-J <njjyl723@gmail.com>
Date: Fri, 4 Oct 2024 21:58:38 +0800
Subject: [PATCH] use cached now to improve read performance

---
 internal/clock/clock.go | 15 +++++++++++++++
 internal/store.go       | 26 +++++++++++++++++++++++++-
 internal/store_test.go  | 39 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 79 insertions(+), 1 deletion(-)

diff --git a/internal/clock/clock.go b/internal/clock/clock.go
index d480c50..5f890a5 100644
--- a/internal/clock/clock.go
+++ b/internal/clock/clock.go
@@ -1,17 +1,32 @@
 package clock
 
 import (
+	"sync/atomic"
 	"time"
 )
 
 type Clock struct {
 	Start time.Time
+	now   atomic.Int64
 }
 
 func (c *Clock) NowNano() int64 {
 	return time.Since(c.Start).Nanoseconds()
 }
 
+func (c *Clock) NowNanoCached() int64 {
+	return c.now.Load()
+}
+
+func (c *Clock) RefreshNowCache() {
+	c.now.Store(c.NowNano())
+}
+
+// used in test only
+func (c *Clock) SetNowCache(n int64) {
+	c.now.Store(n)
+}
+
 func (c *Clock) ExpireNano(ttl time.Duration) int64 {
 	return c.NowNano() + ttl.Nanoseconds()
 }
diff --git a/internal/store.go b/internal/store.go
index b46eab2..b48cc19 100644
--- a/internal/store.go
+++ b/internal/store.go
@@ -220,7 +220,26 @@ func (s *Store[K, V]) getFromShard(key K, hash uint64, shard *Shard[K, V]) (V, b
 	var value V
 	if ok {
 		expire := entry.expire.Load()
-		if expire != 0 && expire <= s.timerwheel.clock.NowNano() {
+		var expired bool
+		if expire != 0 {
+			// Cached now is refreshed every second by ticker.
+			// However, since tickers aren't guaranteed to be precise
+			// https://github.com/golang/go/issues/45632
+			// relax this to 30 seconds. If the entry's expiration time is
+			// less than 30 seconds compared to the cached now,
+			// refetch the accurate now and compare again.
+			nowCached := s.timerwheel.clock.NowNanoCached()
+			if expire-nowCached <= 0 {
+				expired = true
+			} else if expire-nowCached < 30*1e9 {
+				now := s.timerwheel.clock.NowNano()
+				expired = (expire-now <= 0)
+			} else {
+				expired = false
+			}
+		}
+
+		if expired {
 			ok = false
 			s.policy.misses.Add(1)
 		} else {
@@ -580,6 +599,10 @@ func (s *Store[K, V]) drainWrite() {
 }
 
 func (s *Store[K, V]) maintenance() {
+	s.mlock.Lock()
+	s.timerwheel.clock.RefreshNowCache()
+	s.mlock.Unlock()
+
 	go func() {
 		s.mlock.Lock()
 		s.maintenanceTicker = time.NewTicker(time.Second)
@@ -592,6 +615,7 @@ func (s *Store[K, V]) maintenance() {
 				return
 			case <-s.maintenanceTicker.C:
 				s.mlock.Lock()
+				s.timerwheel.clock.RefreshNowCache()
 				if s.closed {
 					s.mlock.Unlock()
 					return
diff --git a/internal/store_test.go b/internal/store_test.go
index 1c47830..b5b3511 100644
--- a/internal/store_test.go
+++ b/internal/store_test.go
@@ -163,3 +163,42 @@ func TestStore_PolicyCounter(t *testing.T) {
 	require.Equal(t, uint64(1600), store.policy.hits.Value())
 	require.Equal(t, uint64(1600), store.policy.misses.Value())
 }
+
+func TestStore_GetExpire(t *testing.T) {
+	store := NewStore[int, int](1000, false, nil, nil, nil, 0, 0, nil)
+	defer store.Close()
+
+	_, i := store.index(123)
+	fakeNow := store.timerwheel.clock.NowNano() - 100*10e9
+	testNow := store.timerwheel.clock.NowNano()
+	entry := &Entry[int, int]{
+		key:   123,
+		value: 123,
+	}
+	entry.expire.Store(fakeNow)
+
+	store.shards[i].hashmap[123] = entry
+	store.mlock.Lock()
+
+	// already exprired
+	store.timerwheel.clock.SetNowCache(fakeNow + 1)
+	_, ok := store.Get(123)
+	require.False(t, ok)
+
+	// use cached now, not expire
+	store.timerwheel.clock.SetNowCache(fakeNow - 31*10e9)
+	v, ok := store.Get(123)
+	require.True(t, ok)
+	require.Equal(t, 123, v)
+
+	// less than 30 seconds and not expired, use real now
+	store.timerwheel.clock.SetNowCache(fakeNow - 1)
+	_, ok = store.Get(123)
+	require.False(t, ok)
+	store.mlock.Unlock()
+
+	// ticker refresh cached now
+	time.Sleep(1200 * time.Millisecond)
+	cachedNow := store.timerwheel.clock.NowNanoCached()
+	require.True(t, cachedNow > testNow)
+}