Log file descriptor limits at startup (#3567)

Fixes #3564

cmd/prometheus/fdlimits_default.go

+// Copyright 2017 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build !windows
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"syscall"
+)
+
+// FdLimits returns the soft and hard limits for file descriptors
+func FdLimits() string {
+	flimit := syscall.Rlimit{}
+	err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &flimit)
+	if err != nil {
+		log.Fatal("Error!")
+	}
+	return fmt.Sprintf("(soft=%d, hard=%d)", flimit.Cur, flimit.Max)
+}

cmd/prometheus/fdlimits_windows.go

+// Copyright 2017 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build windows
+
+package main
+
+// FdLimits not supported on Windows
+func FdLimits() string {
+	return "N/A"
+}

cmd/prometheus/main.go

@@ -220,6 +220,7 @@ func main() {
 	level.Info(logger).Log("msg", "Starting Prometheus", "version", version.Info())
 	level.Info(logger).Log("build_context", version.BuildContext())
 	level.Info(logger).Log("host_details", Uname())
+	level.Info(logger).Log("fd_limits", FdLimits())
 	var (
 		localStorage  = &tsdb.ReadyStorage{}