app/vmselect: limit the default value for -search.maxConcurrentRequests, so it plays well on systems with more than 16 vCPUs

A single heavy request can saturate all the available CPUs, so let's limit the number of concurrent requests to lower value.
This will give more chances for executing insert path.

app/vmselect/main.go

@@ -21,10 +21,25 @@ import (
 
 var (
 	deleteAuthKey         = flag.String("deleteAuthKey", "", "authKey for metrics' deletion via /api/v1/admin/tsdb/delete_series")
-	maxConcurrentRequests = flag.Int("search.maxConcurrentRequests", runtime.GOMAXPROCS(-1)*2, "The maximum number of concurrent search requests. It shouldn't exceed 2*vCPUs for better performance. See also -search.maxQueueDuration")
+	maxConcurrentRequests = flag.Int("search.maxConcurrentRequests", getDefaultMaxConcurrentRequests(), "The maximum number of concurrent search requests. "+
-	maxQueueDuration      = flag.Duration("search.maxQueueDuration", 10*time.Second, "The maximum time the request waits for execution when -search.maxConcurrentRequests limit is reached")
+		"It shouldn't be high, since a single request can saturate all the CPU cores. See also `-search.maxQueueDuration`")
+	maxQueueDuration = flag.Duration("search.maxQueueDuration", 10*time.Second, "The maximum time the request waits for execution when `-search.maxConcurrentRequests` limit is reached")
 )
 
+func getDefaultMaxConcurrentRequests() int {
+	n := runtime.GOMAXPROCS(-1)
+	if n <= 4 {
+		n *= 2
+	}
+	if n > 16 {
+		// A single request can saturate all the CPU cores, so there is no sense
+		// in allowing higher number of concurrent requests - they will just contend
+		// for unavailable CPU time.
+		n = 16
+	}
+	return n
+}
+
 // Init initializes vmselect
 func Init() {
 	tmpDirPath := *vmstorage.DataPath + "/tmp"