From e44e753e66e98f7ad4a624e9696aa85c2b9c005e Mon Sep 17 00:00:00 2001
From: Xiang Li <xiangli.cs@gmail.com>
Date: Thu, 4 Feb 2016 13:06:27 -0800
Subject: [PATCH] pkg/idutil: reduce conflict rate from 1% to 0.005%

Perviously, we only use 8bits from member identification
in id generation. The conflict rate is A(256,3)/256^3, which
is around 1%. Now we use 16bites to reduce the rate to 0.005%.

We can attach the full member id into id generation if needed...
---
 etcdserver/server.go  |  2 +-
 pkg/idutil/id.go      | 12 ++++++------
 pkg/idutil/id_test.go |  4 ++--
 3 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/etcdserver/server.go b/etcdserver/server.go
index 020ed3687..ab737c167 100644
--- a/etcdserver/server.go
+++ b/etcdserver/server.go
@@ -358,7 +358,7 @@ func NewServer(cfg *ServerConfig) (*EtcdServer, error) {
 		lstats:        lstats,
 		SyncTicker:    time.Tick(500 * time.Millisecond),
 		peerRt:        prt,
-		reqIDGen:      idutil.NewGenerator(uint8(id), time.Now()),
+		reqIDGen:      idutil.NewGenerator(uint16(id), time.Now()),
 		forceVersionC: make(chan struct{}),
 		msgSnapC:      make(chan raftpb.Message, maxInFlightMsgSnap),
 	}
diff --git a/pkg/idutil/id.go b/pkg/idutil/id.go
index 0ba570cff..50d80d963 100644
--- a/pkg/idutil/id.go
+++ b/pkg/idutil/id.go
@@ -24,7 +24,7 @@ import (
 
 const (
 	tsLen     = 5 * 8
-	cntLen    = 2 * 8
+	cntLen    = 8
 	suffixLen = tsLen + cntLen
 )
 
@@ -32,7 +32,7 @@ const (
 // High order byte is memberID, next 5 bytes are from timestamp,
 // and low order 2 bytes are 0s.
 // | prefix   | suffix              |
-// | 1 byte   | 5 bytes   | 2 bytes |
+// | 2 bytes  | 5 bytes   | 1 byte  |
 // | memberID | timestamp | cnt     |
 //
 // The timestamp 5 bytes is different when the machine is restart
@@ -42,16 +42,16 @@ const (
 // The count field may overflow to timestamp field, which is intentional.
 // It helps to extend the event window to 2^56. This doesn't break that
 // id generated after restart is unique because etcd throughput is <<
-// 65536req/ms.
+// 256req/ms(250k reqs/second).
 type Generator struct {
 	mu sync.Mutex
-	// high order byte
+	// high order 2 bytes
 	prefix uint64
-	// low order 7 bytes
+	// low order 6 bytes
 	suffix uint64
 }
 
-func NewGenerator(memberID uint8, now time.Time) *Generator {
+func NewGenerator(memberID uint16, now time.Time) *Generator {
 	prefix := uint64(memberID) << suffixLen
 	unixMilli := uint64(now.UnixNano()) / uint64(time.Millisecond/time.Nanosecond)
 	suffix := lowbit(unixMilli, tsLen) << cntLen
diff --git a/pkg/idutil/id_test.go b/pkg/idutil/id_test.go
index b5249883f..19e2a8ff1 100644
--- a/pkg/idutil/id_test.go
+++ b/pkg/idutil/id_test.go
@@ -22,7 +22,7 @@ import (
 func TestNewGenerator(t *testing.T) {
 	g := NewGenerator(0x12, time.Unix(0, 0).Add(0x3456*time.Millisecond))
 	id := g.Next()
-	wid := uint64(0x1200000034560001)
+	wid := uint64(0x12000000345601)
 	if id != wid {
 		t.Errorf("id = %x, want %x", id, wid)
 	}
@@ -45,7 +45,7 @@ func TestNewGeneratorUnique(t *testing.T) {
 
 func TestNext(t *testing.T) {
 	g := NewGenerator(0x12, time.Unix(0, 0).Add(0x3456*time.Millisecond))
-	wid := uint64(0x1200000034560001)
+	wid := uint64(0x12000000345601)
 	for i := 0; i < 1000; i++ {
 		id := g.Next()
 		if id != wid+uint64(i) {