promql: force mmap of head chunks in BenchmarkRangeQuery

Otherwise we have a highly unusual situation of over 100 chunks
in the headChunks list of each series, which heavily skews
performance.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

promql/bench_test.go

@@ -66,6 +66,8 @@ func setupRangeQueryTestData(stor *teststorage.TestStorage, _ *Engine, interval,
 			return err
 		}
 	}
+	stor.DB.ForceHeadMMap() // Ensure we have at most one head chunk for every series.
+	stor.DB.Compact()
 	return nil
 }
 
@@ -222,6 +224,7 @@ func rangeQueryCases() []benchCase {
 
 func BenchmarkRangeQuery(b *testing.B) {
 	stor := teststorage.New(b)
+	stor.DB.DisableCompactions() // Don't want auto-compaction disrupting timings.
 	defer stor.Close()
 	opts := EngineOpts{
 		Logger:     nil,

tsdb/db.go

@@ -1797,6 +1797,11 @@ func (db *DB) EnableCompactions() {
 	level.Info(db.logger).Log("msg", "Compactions enabled")
 }
 
+// ForceHeadMMap is intended for use only in tests and benchmarks.
+func (db *DB) ForceHeadMMap() {
+	db.head.mmapHeadChunks()
+}
+
 // Snapshot writes the current data to the directory. If withHead is set to true it
 // will create a new block containing all data that's currently in the memory buffer/WAL.
 func (db *DB) Snapshot(dir string, withHead bool) error {