From 6efcf9c982684765fcfef416fdb43c012b14cb3e Mon Sep 17 00:00:00 2001
From: Klaus Post <klauspost@gmail.com>
Date: Wed, 5 Jul 2023 10:40:45 -0700
Subject: [PATCH] Do lockless last minute latency metrics (#17576)

Collect metrics in one second and accumulate lockless before sending upstream.
---
 cmd/last-minute.go              | 15 ++---------
 cmd/xl-storage-disk-id-check.go | 48 ++++++++++++++++++++++++++-------
 2 files changed, 41 insertions(+), 22 deletions(-)

diff --git a/cmd/last-minute.go b/cmd/last-minute.go
index e21dfab0a..aff5965ea 100644
--- a/cmd/last-minute.go
+++ b/cmd/last-minute.go
@@ -90,16 +90,6 @@ func (a *AccElem) add(dur time.Duration) {
 	a.N++
 }
 
-// Add a duration to a single element.
-func (a *AccElem) addSize(dur time.Duration, sz int64) {
-	if dur < 0 {
-		dur = 0
-	}
-	a.Total += int64(dur)
-	a.Size += sz
-	a.N++
-}
-
 // Merge b into a.
 func (a *AccElem) merge(b AccElem) {
 	a.N += b.N
@@ -156,11 +146,10 @@ func (l *lastMinuteLatency) add(t time.Duration) {
 }
 
 // Add  a new duration data
-func (l *lastMinuteLatency) addSize(t time.Duration, sz int64) {
-	sec := time.Now().Unix()
+func (l *lastMinuteLatency) addAll(sec int64, a AccElem) {
 	l.forwardTo(sec)
 	winIdx := sec % 60
-	l.Totals[winIdx].addSize(t, sz)
+	l.Totals[winIdx].merge(a)
 	l.LastSec = sec
 }
 
diff --git a/cmd/xl-storage-disk-id-check.go b/cmd/xl-storage-disk-id-check.go
index 8c2eaddee..ecc62c033 100644
--- a/cmd/xl-storage-disk-id-check.go
+++ b/cmd/xl-storage-disk-id-check.go
@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"io"
 	"math/rand"
+	"runtime"
 	"strconv"
 	"strings"
 	"sync"
@@ -105,28 +106,57 @@ func (p *xlStorageDiskIDCheck) getMetrics() DiskMetrics {
 	return m.(DiskMetrics)
 }
 
+// lockedLastMinuteLatency accumulates totals lockless for each second.
 type lockedLastMinuteLatency struct {
-	sync.Mutex
+	cachedSec int64
+	cached    atomic.Pointer[AccElem]
+	mu        sync.Mutex
+	init      sync.Once
 	lastMinuteLatency
 }
 
 func (e *lockedLastMinuteLatency) add(value time.Duration) {
-	e.Lock()
-	defer e.Unlock()
-	e.lastMinuteLatency.add(value)
+	e.addSize(value, 0)
 }
 
 // addSize will add a duration and size.
 func (e *lockedLastMinuteLatency) addSize(value time.Duration, sz int64) {
-	e.Lock()
-	defer e.Unlock()
-	e.lastMinuteLatency.addSize(value, sz)
+	// alloc on every call, so we have a clean entry to swap in.
+	t := time.Now().Unix()
+	e.init.Do(func() {
+		e.cached.Store(&AccElem{})
+		atomic.StoreInt64(&e.cachedSec, t)
+	})
+	acc := e.cached.Load()
+	if lastT := atomic.LoadInt64(&e.cachedSec); lastT != t {
+		// Check if lastT was changed by someone else.
+		if atomic.CompareAndSwapInt64(&e.cachedSec, lastT, t) {
+			// Now we swap in a new.
+			newAcc := &AccElem{}
+			old := e.cached.Swap(newAcc)
+			var a AccElem
+			a.Size = atomic.LoadInt64(&old.Size)
+			a.Total = atomic.LoadInt64(&old.Total)
+			a.N = atomic.LoadInt64(&old.N)
+			e.mu.Lock()
+			e.lastMinuteLatency.addAll(t-1, a)
+			e.mu.Unlock()
+			acc = newAcc
+		} else {
+			// We may be able to grab the new accumulator by yielding.
+			runtime.Gosched()
+			acc = e.cached.Load()
+		}
+	}
+	atomic.AddInt64(&acc.N, 1)
+	atomic.AddInt64(&acc.Total, int64(value))
+	atomic.AddInt64(&acc.Size, sz)
 }
 
 // total returns the total call count and latency for the last minute.
 func (e *lockedLastMinuteLatency) total() AccElem {
-	e.Lock()
-	defer e.Unlock()
+	e.mu.Lock()
+	defer e.mu.Unlock()
 	return e.lastMinuteLatency.getTotal()
 }