restrict number of disks used for scanning buckets upto GOMAXPROCS (#15492)

control scanner parallelism to avoid higher CPU
usage on nodes that have more drives but an old CPU.

cmd/erasure.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"math/rand"
 	"os"
+	"runtime"
 	"sort"
 	"sync"
 	"time"
@@ -439,6 +440,13 @@ func (er erasureObjects) nsScanner(ctx context.Context, buckets []BucketInfo, bf
 	r := rand.New(rand.NewSource(time.Now().UnixNano()))
 	r.Shuffle(len(disks), func(i, j int) { disks[i], disks[j] = disks[j], disks[i] })
 
+	// Restrict parallelism for disk usage scanner
+	// upto GOMAXPROCS if GOMAXPROCS is < len(disks)
+	maxProcs := runtime.GOMAXPROCS(0)
+	if maxProcs < len(disks) {
+		disks = disks[:maxProcs]
+	}
+
 	// Start one scanner per disk
 	var wg sync.WaitGroup
 	wg.Add(len(disks))