From dc8f50559534a0820823c27ded103f3cee4b2af4 Mon Sep 17 00:00:00 2001
From: Bryan Boreham <bjboreham@gmail.com>
Date: Wed, 7 Jul 2021 14:28:20 +0100
Subject: [PATCH] tsdb: coalesce lock/unlock operations for append (#9061)

Fetch the low watermark value under the same lock as we need for the
appender, rather than releasing then re-aquiring a lock on the same
Mutex.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>
---
 tsdb/head.go           | 3 +--
 tsdb/isolation.go      | 9 +++++++--
 tsdb/isolation_test.go | 6 ++----
 3 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/tsdb/head.go b/tsdb/head.go
index 1fce0368fc..02ae2a140d 100644
--- a/tsdb/head.go
+++ b/tsdb/head.go
@@ -1232,8 +1232,7 @@ func (h *Head) Appender(_ context.Context) storage.Appender {
 }
 
 func (h *Head) appender() *headAppender {
-	appendID := h.iso.newAppendID()
-	cleanupAppendIDsBelow := h.iso.lowWatermark()
+	appendID, cleanupAppendIDsBelow := h.iso.newAppendID()
 
 	// Allocate the exemplars buffer only if exemplars are enabled.
 	var exemplarsBuf []exemplarWithSeriesRef
diff --git a/tsdb/isolation.go b/tsdb/isolation.go
index cee6899dd7..51302b23d4 100644
--- a/tsdb/isolation.go
+++ b/tsdb/isolation.go
@@ -86,6 +86,10 @@ func newIsolation() *isolation {
 func (i *isolation) lowWatermark() uint64 {
 	i.appendMtx.RLock() // Take appendMtx first.
 	defer i.appendMtx.RUnlock()
+	return i.lowWatermarkLocked()
+}
+
+func (i *isolation) lowWatermarkLocked() uint64 {
 	i.readMtx.RLock()
 	defer i.readMtx.RUnlock()
 	if i.readsOpen.prev != i.readsOpen {
@@ -122,7 +126,8 @@ func (i *isolation) State() *isolationState {
 
 // newAppendID increments the transaction counter and returns a new transaction
 // ID. The first ID returned is 1.
-func (i *isolation) newAppendID() uint64 {
+// Also returns the low watermark, to keep lock/unlock operations down
+func (i *isolation) newAppendID() (uint64, uint64) {
 	i.appendMtx.Lock()
 	defer i.appendMtx.Unlock()
 
@@ -138,7 +143,7 @@ func (i *isolation) newAppendID() uint64 {
 	i.appendsOpenList.prev = app
 
 	i.appendsOpen[app.appendID] = app
-	return app.appendID
+	return app.appendID, i.lowWatermarkLocked()
 }
 
 func (i *isolation) lastAppendID() uint64 {
diff --git a/tsdb/isolation_test.go b/tsdb/isolation_test.go
index 3a90547b20..79ae21435a 100644
--- a/tsdb/isolation_test.go
+++ b/tsdb/isolation_test.go
@@ -35,8 +35,7 @@ func BenchmarkIsolation(b *testing.B) {
 					<-start
 
 					for i := 0; i < b.N; i++ {
-						appendID := iso.newAppendID()
-						_ = iso.lowWatermark()
+						appendID, _ := iso.newAppendID()
 
 						iso.closeAppend(appendID)
 					}
@@ -66,8 +65,7 @@ func BenchmarkIsolationWithState(b *testing.B) {
 					<-start
 
 					for i := 0; i < b.N; i++ {
-						appendID := iso.newAppendID()
-						_ = iso.lowWatermark()
+						appendID, _ := iso.newAppendID()
 
 						iso.closeAppend(appendID)
 					}