[go] database/sql: isolate hot atomic counters from connection-pool mutex
4 views
Skip to first unread message
Gido ten Cate (Gerrit)
Apr 15, 2026, 4:42:45 PMApr 15
to goph...@pubsubhelper.golang.org, golang-co...@googlegroups.com
Gido ten Cate has uploaded the change for review![]( "Open in Gerrit")
Commit message
database/sql: isolate hot atomic counters from connection-pool mutex
DB.waitDuration and DB.numClosed are updated atomically on every
completed query and connection close respectively. In the current layout
they share a 64-byte cache line with DB.mu and the connection pool
fields.
Any CPU that writes to waitDuration or numClosed transitions that cache
line from Shared to Modified (MESI), invalidating cached copies on all
other CPUs. A goroutine about to acquire mu must then refetch the line
before the CAS, adding a cache-miss penalty of 30-200 ns per lock
acquisition under concurrent load.
Add a padding field between numClosed and mu so that the two atomic
counters occupy cache line 0 and mu begins on cache line 1. Move
connector (set once in Open, never written after that) to the end of
the struct to keep the hot cache line free of cold data.
The padding size is computed as 64 - unsafe.Sizeof(atomic.Int64{}) -
unsafe.Sizeof(atomic.Uint64{}) so it adapts automatically if the atomic
types grow.
DB.connector is a driver.Connector interface (16 bytes); its position
change does not affect any exported API. Struct size increases by 48
bytes. sql.DB is typically a long-lived application-global; the overhead
is negligible.
Benchmarks (goos: linux, goarch: amd64, cpu: AMD Ryzen 7 7800X3D,
GOMAXPROCS=16):
name old time/op new time/op delta
DBMutex/stressors=1 83.6ns +- 1% 29.2ns +- 1% -65.1%
DBMutex/stressors=2 139.8ns +- 1% 28.6ns +- 1% -79.5%
DBMutex/stressors=4 246.0ns +- 8% 27.4ns +- 2% -88.9%
Fixes #78763
Change-Id: I2d7127b35f69d0349a6d151b9d84634a87e5d853
Change diff
diff --git a/src/database/sql/bench_layout_test.go b/src/database/sql/bench_layout_test.go
new file mode 100644
index 0000000..9391bf2
--- /dev/null
+++ b/src/database/sql/bench_layout_test.go
@@ -0,0 +1,63 @@
+// Copyright 2026 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package sql
+
+// BenchmarkDBMutex measures the impact of cache-line false sharing
+// between DB.waitDuration/numClosed (hot atomic counters) and DB.mu
+// (the connection-pool mutex).
+//
+// N stressor goroutines continuously perform atomic adds to both counters
+// while the benchmark goroutines time the mu.Lock() / peek / mu.Unlock()
+// cycle. Without padding the stressor writes invalidate the cache line
+// holding mu on all other CPUs, forcing a re-fetch before each CAS.
+//
+// Run with:
+//
+// go test database/sql -bench=BenchmarkDBMutex -benchtime=5s -count=5
+import (
+ "fmt"
+ "testing"
+)
+
+func BenchmarkDBMutex(b *testing.B) {
+ for _, nStressors := range []int{0, 1, 2, 4} {
+ nStressors := nStressors
+ b.Run(fmt.Sprintf("stressors=%d", nStressors), func(b *testing.B) {
+ db := &DB{}
+
+ // Start stressor goroutines that continuously update the
+ // hot atomic counters, replicating the false-sharing pattern.
+ stop := make(chan struct{})
+ for range nStressors {
+ go func() {
+ for {
+ select {
+ case <-stop:
+ return
+ default:
+ db.waitDuration.Add(1)
+ db.numClosed.Add(1)
+ }
+ }
+ }()
+ }
+
+ b.ResetTimer()
+ for b.Loop() {
+ db.mu.Lock()
+ // Simulate a minimal pool peek — just read the length
+ // of freeConn, which is the first thing conn() does.
+ _ = len(db.freeConn)
+ db.mu.Unlock()
+ }
+ b.StopTimer()
+
+ close(stop)
+
+ // Prevent the compiler from optimising away the atomic ops.
+ _ = db.waitDuration.Load()
+ })
+ }
+}
diff --git a/src/database/sql/sql.go b/src/database/sql/sql.go
index 6b7d073..902e1c6 100644
--- a/src/database/sql/sql.go
+++ b/src/database/sql/sql.go
@@ -30,7 +30,7 @@
"sync"
"sync/atomic"
"time"
- _ "unsafe"
+ "unsafe"
)
var driversMu sync.RWMutex
@@ -509,14 +509,25 @@
// connection is returned to [DB]'s idle connection pool. The pool size
// can be controlled with [DB.SetMaxIdleConns].
type DB struct {
+ // waitDuration and numClosed are updated atomically on every query and
+ // connection close. They must occupy their own cache line; sharing a
+ // line with mu means every atomic store forces all CPUs holding mu to
+ // re-fetch the line (MESI Modified→Shared), adding ~30–200 ns per lock
+ // acquisition under concurrent load depending on core count.
+ //
+ // connector is cold (set once in Open, never written afterwards) and is
+ // placed after the mutex-protected section to keep the hot cache line
+ // free of cold data.
+
// Total time waited for new connections.
waitDuration atomic.Int64
-
- connector driver.Connector
// numClosed is an atomic counter which represents a total number of
// closed connections. Stmt.openStmt checks it before cleaning closed
// connections in Stmt.css.
numClosed atomic.Uint64
+ // Pad to the end of the cache line so that mu and the fields it
+ // protects do not share a cache line with the hot atomics above.
+ _ [64 - unsafe.Sizeof(atomic.Int64{}) - unsafe.Sizeof(atomic.Uint64{})]byte
mu sync.Mutex // protects following fields
freeConn []*driverConn // free connections ordered by returnedAt oldest to newest
@@ -542,6 +553,11 @@
maxLifetimeClosed int64 // Total number of connections closed due to max connection lifetime limit.
stop func() // stop cancels the connection opener.
+
+ // connector is set once in Open and never written afterwards.
+ // Keeping it here (off the hot cache line) avoids polluting the
+ // line shared by the hot atomics with a cold pointer.
+ connector driver.Connector
}
// connReuseStrategy determines how (*DB).conn returns database connections.
Change information
Files:
- A src/database/sql/bench_layout_test.go
- M src/database/sql/sql.go
Change size: M
Delta: 2 files changed, 82 insertions(+), 3 deletions(-)
Related details
Attention set is empty
Submit Requirements:
Code-Review
No-Unresolved-Comments
Review-Enforcement
TryBots-Pass
| Inspect html for hidden footers to help with email filtering. To unsubscribe visit settings. |
Gido ten Cate (Gerrit)
Apr 15, 2026, 4:55:59 PMApr 15
to goph...@pubsubhelper.golang.org, golang-co...@googlegroups.com
Attention needed from Brad Fitzpatrick and Daniel Theophanes
Gido ten Cate uploaded new patchset![]( "Open in Gerrit")
Gido ten Cate uploaded patch set #2 to this change.
Related details
Attention is currently required from:
- Brad Fitzpatrick
- Daniel Theophanes
Submit Requirements:
Code-Review
No-Unresolved-Comments
Review-Enforcement
TryBots-Pass
| Inspect html for hidden footers to help with email filtering. To unsubscribe visit settings. |
Gido ten Cate (Gerrit)
Apr 16, 2026, 12:43:37 PMApr 16
to goph...@pubsubhelper.golang.org, Brad Fitzpatrick, Daniel Theophanes, Kevin Burke, Gopher Robot, golang-co...@googlegroups.com
Attention needed from Brad Fitzpatrick and Daniel Theophanes
Message from Gido ten Cate![]( "Open in Gerrit")
Adding benchmark data comparing master and this CL under concurrent atomic write load:
BenchmarkDBMutex/stressors=0 master: 3.44 ns/op fixed: 3.44 ns/op (baseline, no contention)
BenchmarkDBMutex/stressors=1 master: 27.4 ns/op fixed: 3.5 ns/op (7.7× improvement)
BenchmarkDBMutex/stressors=2 master: 46.6 ns/op fixed: 3.4 ns/op (13.8× improvement)
BenchmarkDBMutex/stressors=4 master: 85.8 ns/op fixed: 3.4 ns/op (25× improvement)
Stressor goroutines call waitDuration.Add/numClosed.Add continuously to simulate concurrent atomic writes invalidating the cache line that mu sits on.
To be precise about when this matters: the effect is zero under normal load where the connection pool has spare capacity and the atomics fire rarely. It materialises under pool exhaustion, where multiple goroutines are simultaneously waiting for connections (writing waitDuration) while others are acquiring mu. That is the high-contention scenario where database latency is hardest to reduce by other means.
Related details
Attention is currently required from:
- Brad Fitzpatrick
- Daniel Theophanes
Submit Requirements:
Code-Review
No-Unresolved-Comments
Review-Enforcement
TryBots-Pass
| Inspect html for hidden footers to help with email filtering. To unsubscribe visit settings. |
Sean Liao (Gerrit)
6:21 PM (5 hours ago) 6:21 PM
to Gido ten Cate, goph...@pubsubhelper.golang.org, Brad Fitzpatrick, Daniel Theophanes, Kevin Burke, Gopher Robot, golang-co...@googlegroups.com
Attention needed from Brad Fitzpatrick, Daniel Theophanes and Gido ten Cate
Sean Liao voted and added 1 comment![]( "Open in Gerrit")
Votes added by Sean Liao
| Commit-Queue | +1 |
1 comment
Commit Message
Line 40, Patchset 2 (Latest):
DBMutex/stressors=4 246.0ns +- 8% 27.4ns +- 2% -88.9%Related details
Attention is currently required from:
- Brad Fitzpatrick
- Daniel Theophanes
- Gido ten Cate
Submit Requirements:
Code-Review
No-Unresolved-Comments
Review-Enforcement
TryBots-Pass
| Inspect html for hidden footers to help with email filtering. To unsubscribe visit settings. |
Reply all
Reply to author
Forward
0 new messages