perf(weed/storage/needle): intern the stored ttl values (#10611)

The master decodes a TTL per volume in every heartbeat and keeps it for the
volume's lifetime, so a cluster using TTLs carries one two-byte object per
volume replica where at most 256 counts times 7 units exist. Share them, and
decode the uint32 form directly instead of staging it through a byte slice.

Clusters that set no TTL are unaffected; that path already returned the shared
EMPTY_TTL.

BenchmarkSyncDataNodeRegistration/100000Volumes, volumes carrying a ttl
  600600 allocs/op -> 500597 allocs/op

weed/storage/needle/volume_ttl.go

@@ -88,20 +88,39 @@ func fitTtlCount(count int, unit byte) *TTL {
 	return EMPTY_TTL
 }
 
-// read stored bytes to a ttl
-func LoadTTLFromBytes(input []byte) (t *TTL) {
-	if input[0] == 0 && input[1] == 0 {
+// storedTTLs interns every count paired with a defined unit. The master decodes
+// one per volume in every heartbeat and keeps it for the volume's lifetime, so
+// sharing spares a cluster with millions of volume replicas a distinct two-byte
+// object for each.
+//
+// Entries are immutable. Callers must not write through the returned pointer.
+var storedTTLs = func() (table [256][Year + 1]TTL) {
+	for count := range table {
+		for unit := range table[count] {
+			table[count][unit] = TTL{Count: byte(count), Unit: byte(unit)}
+		}
+	}
+	return table
+}()
+
+func loadTTL(count, unit byte) *TTL {
+	if count == 0 && unit == 0 {
 		return EMPTY_TTL
 	}
-	return &TTL{Count: input[0], Unit: input[1]}
+	if unit <= Year {
+		return &storedTTLs[count][unit]
+	}
+	return &TTL{Count: count, Unit: unit}
+}
+
+// read stored bytes to a ttl
+func LoadTTLFromBytes(input []byte) (t *TTL) {
+	return loadTTL(input[0], input[1])
 }
 
 // read stored bytes to a ttl
 func LoadTTLFromUint32(ttl uint32) (t *TTL) {
-	input := make([]byte, 2)
-	input[1] = byte(ttl)
-	input[0] = byte(ttl >> 8)
-	return LoadTTLFromBytes(input)
+	return loadTTL(byte(ttl>>8), byte(ttl))
 }
 
 // save stored bytes to an output with 2 bytes

weed/storage/needle/volume_ttl_test.go

@@ -73,3 +73,26 @@ func TestTTLReadWrite(t *testing.T) {
 	}
 
 }
+
+func TestLoadTTLIsInterned(t *testing.T) {
+	for count := 0; count < 256; count++ {
+		for unit := 0; unit < 256; unit++ {
+			got := LoadTTLFromBytes([]byte{byte(count), byte(unit)})
+			if got.Count != byte(count) || got.Unit != byte(unit) {
+				if count == 0 && unit == 0 {
+					continue
+				}
+				t.Fatalf("count %d unit %d: got %+v", count, unit, got)
+			}
+			if fromUint32 := LoadTTLFromUint32(got.ToUint32()); count != 0 && *fromUint32 != *got {
+				t.Fatalf("count %d unit %d: uint32 round trip gave %+v", count, unit, fromUint32)
+			}
+		}
+	}
+	if LoadTTLFromBytes([]byte{3, Day}) != LoadTTLFromBytes([]byte{3, Day}) {
+		t.Error("expected repeated loads of the same stored ttl to share one value")
+	}
+	if LoadTTLFromBytes([]byte{0, 0}) != EMPTY_TTL {
+		t.Error("expected a zero ttl to stay EMPTY_TTL")
+	}
+}