Add automatic Conter64 wrapping

Automatically wrap Counter64 PDU values every 2^53 to avoid float64 precision loss. Add a command line flag to enable/disable this feature, enable by default. Signed-off-by: Ben Kochie <[email protected]>
prometheus · Nov 2, 2018 · 3d47144 · 3d47144
1 parent 5f4dfb4
commit 3d47144
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Showing 4 changed files with 48 additions and 1 deletion.
diff --git a/README.md b/README.md
@@ -62,3 +62,10 @@ scrape_configs:
 This setup allows Prometheus to provide scheduling and service discovery, as
 unlike all other exporters running an exporter on the machine from which we are
 getting the metrics from is not possible.
+
+## Large counter value handling
+
+In order to provide accurate counters for large Counter64 values, the exporter will automatically
+wrap the value every 2^53 to avoid 64-bit float rounding.
+
+To disable this feature, use the command line flag `--no-wrap-large-counters`.
diff --git a/collector.go b/collector.go
@@ -252,7 +252,12 @@ PduLoop:
 func getPduValue(pdu *gosnmp.SnmpPDU) float64 {
 	switch pdu.Type {
 	case gosnmp.Counter64:
-		return float64(gosnmp.ToBigInt(pdu.Value).Uint64())
+		if *wrapCounters {
+			// Wrap by 2^53
+			return float64(gosnmp.ToBigInt(pdu.Value).Uint64() % 9007199254740992)
+		} else {
+			return float64(gosnmp.ToBigInt(pdu.Value).Uint64())
+		}
 	case gosnmp.OpaqueFloat:
 		return float64(pdu.Value.(float32))
 	case gosnmp.OpaqueDouble:

diff --git a/collector_test.go b/collector_test.go
@@ -21,6 +21,8 @@ import (
 
 	"github.com/prometheus/client_model/go"
 	"github.com/soniah/gosnmp"
+	"github.com/prometheus/common/log"
+	kingpin "gopkg.in/alecthomas/kingpin.v2"
 
 	"github.com/prometheus/snmp_exporter/config"
 )
@@ -462,6 +464,38 @@ func TestGetPduValue(t *testing.T) {
 	}
 }
 
+func TestGetPduLargeValue(t *testing.T) {
+	// Setup default flags and suppress logging.
+	log.AddFlags(kingpin.CommandLine)
+	_, err := kingpin.CommandLine.Parse([]string{"--log.level", "fatal"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	pdu := &gosnmp.SnmpPDU{
+		Value: uint64(19007199254740992),
+		Type:  gosnmp.Counter64,
+	}
+	value := getPduValue(pdu)
+	if value != 992800745259008.0 {
+		t.Fatalf("Got incorrect counter wrapping for Counter64: %v", value)
+	}
+
+	_, err = kingpin.CommandLine.Parse([]string{"--log.level", "fatal", "--no-wrap-large-counters"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	pdu = &gosnmp.SnmpPDU{
+		Value: uint64(19007199254740992),
+		Type:  gosnmp.Counter64,
+	}
+	value = getPduValue(pdu)
+	if value != 19007199254740990.0 {
+		t.Fatalf("Got incorrect rounded float for Counter64: %v", value)
+	}
+}
+
 func TestOidToList(t *testing.T) {
 	cases := []struct {
 		oid    string

diff --git a/main.go b/main.go
@@ -37,6 +37,7 @@ import (
 var (
 	configFile    = kingpin.Flag("config.file", "Path to configuration file.").Default("snmp.yml").String()
 	listenAddress = kingpin.Flag("web.listen-address", "Address to listen on for web interface and telemetry.").Default(":9116").String()
+	wrapCounters  = kingpin.Flag("wrap-large-counters", "Wrap 64-bit counters to avoid floating point rounding.").Default("true").Bool()
 
 	// Metrics about the SNMP exporter itself.
 	snmpDuration = prometheus.NewSummaryVec(