Add feature flag

Signed-off-by: Danny Kopping <danny.kopping@grafana.com>

cmd/prometheus/main.go

@@ -158,6 +158,7 @@ type flagConfig struct {
 	enablePerStepStats         bool
 	enableAutoGOMAXPROCS       bool
 	enableAutoGOMEMLIMIT       bool
+	enableConcurrentRuleEval   bool
 
 	prometheusURL   string
 	corsRegexString string
@@ -204,6 +205,9 @@ func (c *flagConfig) setFeatureListOptions(logger log.Logger) error {
 			case "auto-gomemlimit":
 				c.enableAutoGOMEMLIMIT = true
 				level.Info(logger).Log("msg", "Automatically set GOMEMLIMIT to match Linux container or system memory limit")
+			case "concurrent-rule-eval":
+				c.enableConcurrentRuleEval = true
+				level.Info(logger).Log("msg", "Experimental concurrent rule evaluation enabled.")
 			case "no-default-scrape-port":
 				c.scrape.NoDefaultPort = true
 				level.Info(logger).Log("msg", "No default port will be appended to scrape targets' addresses.")

docs/feature_flags.md

@@ -212,3 +212,13 @@ Enables ingestion of created timestamp. Created timestamps are injected as 0 val
 Currently Prometheus supports created timestamps only on the traditional Prometheus Protobuf protocol (WIP for other protocols). As a result, when enabling this feature, the Prometheus protobuf scrape protocol will be prioritized (See `scrape_config.scrape_protocols` settings for more details).
 
 Besides enabling this feature in Prometheus, created timestamps need to be exposed by the application being scraped.
+
+## Concurrent evaluation of independent rules
+
+`--enable-feature=concurrent-rule-eval`
+
+Rule groups execute concurrently, but the rules within a group execute sequentially; this is because rules can use the
+output of a preceding rule as its input. However, if there is no detectable relationship between rules then there is no
+reason to run them sequentially. This can improve rule reliability at the expense of adding more concurrent query
+load. The number of concurrent rule evaluations can be configured with `--rules.max-concurrent-rule-evals` which is set
+to `4` by default.