Add a minIterationDuration option (grafana#821)

* Add a minIterationDuration option

* Fix a gofmt issue

* Fix minIterationDuration issues from code review

cmd/options.go

@@ -26,14 +26,13 @@ import (
 	"strings"
 	"time"
 
+	"github.com/loadimpact/k6/lib"
 	"github.com/loadimpact/k6/lib/types"
 	"github.com/loadimpact/k6/stats"
-	null "gopkg.in/guregu/null.v3"
-
-	"github.com/loadimpact/k6/lib"
 	"github.com/loadimpact/k6/ui"
 	"github.com/pkg/errors"
 	"github.com/spf13/pflag"
+	null "gopkg.in/guregu/null.v3"
 )
 
 var (
@@ -61,6 +60,7 @@ func optionFlagSet() *pflag.FlagSet {
 	flags.Bool("insecure-skip-tls-verify", false, "skip verification of TLS certificates")
 	flags.Bool("no-connection-reuse", false, "disable keep-alive connections")
 	flags.Bool("no-vu-connection-reuse", false, "don't reuse connections between iterations")
+	flags.Duration("min-iteration-duration", 0, "minimum amount of time k6 will take executing a single iteration")
 	flags.BoolP("throw", "w", false, "throw warnings (like failed http requests) as errors")
 	flags.StringSlice("blacklist-ip", nil, "blacklist an `ip range` from being called")
 	flags.StringSlice("summary-trend-stats", nil, "define `stats` for trend metrics (response times), one or more as 'avg,p(95),...'")
@@ -86,6 +86,7 @@ func getOptions(flags *pflag.FlagSet) (lib.Options, error) {
 		InsecureSkipTLSVerify: getNullBool(flags, "insecure-skip-tls-verify"),
 		NoConnectionReuse:     getNullBool(flags, "no-connection-reuse"),
 		NoVUConnectionReuse:   getNullBool(flags, "no-vu-connection-reuse"),
+		MinIterationDuration:  getNullDuration(flags, "min-iteration-duration"),
 		Throw:                 getNullBool(flags, "throw"),
 		DiscardResponseBodies: getNullBool(flags, "discard-response-bodies"),
 		// Default values for options without CLI flags:

core/engine_test.go

@@ -915,3 +915,54 @@ func TestEmittedMetricsWhenScalingDown(t *testing.T) {
 	durationSum := getMetricSum(collector, metrics.IterationDuration.Name)
 	assert.InDelta(t, 1.7, durationSum/(1000*durationCount), 0.1)
 }
+
+func TestMinIterationDuration(t *testing.T) {
+	t.Parallel()
+
+	runner, err := js.New(
+		&lib.SourceData{Filename: "/script.js", Data: []byte(`
+		import { Counter } from "k6/metrics";
+
+		let testCounter = new Counter("testcounter");
+
+		export let options = {
+			minIterationDuration: "1s",
+			vus: 2,
+			vusMax: 2,
+			duration: "1.9s",
+		};
+
+		export default function () {
+			testCounter.add(1);
+		};`)},
+		afero.NewMemMapFs(),
+		lib.RuntimeOptions{},
+	)
+	require.NoError(t, err)
+
+	engine, err := NewEngine(local.New(runner), runner.GetOptions())
+	require.NoError(t, err)
+
+	collector := &dummy.Collector{}
+	engine.Collectors = []lib.Collector{collector}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	errC := make(chan error)
+	go func() { errC <- engine.Run(ctx) }()
+
+	select {
+	case <-time.After(10 * time.Second):
+		cancel()
+		t.Fatal("Test timed out")
+	case err := <-errC:
+		cancel()
+		require.NoError(t, err)
+		require.False(t, engine.IsTainted())
+	}
+
+	// Only 2 full iterations are expected to be completed due to the 1 second minIterationDuration
+	assert.Equal(t, 2.0, getMetricSum(collector, metrics.Iterations.Name))
+
+	// But we expect the custom counter to be added to 4 times
+	assert.Equal(t, 4.0, getMetricSum(collector, "testcounter"))
+}

js/runner.go

@@ -440,5 +440,14 @@ func (u *VU) runFn(ctx context.Context, group *lib.Group, fn goja.Callable, args
 
 	state.Samples <- u.Dialer.GetTrail(startTime, endTime, isFullIteration, stats.IntoSampleTags(&tags))
 
+	// If MinIterationDuration is specified and the iteration wasn't cancelled
+	// and was less than it, sleep for the remainder
+	if isFullIteration && state.Options.MinIterationDuration.Valid {
+		durationDiff := time.Duration(state.Options.MinIterationDuration.Duration) - endTime.Sub(startTime)
+		if durationDiff > 0 {
+			time.Sleep(durationDiff)
+		}
+	}
+
 	return v, state, err
 }

js/runner_test.go

@@ -1259,7 +1259,7 @@ func TestHTTPRequestInInitContext(t *testing.T) {
 					import http from "k6/http";
 					let res = http.get("HTTPBIN_URL/");
 					export default function() {
-						console.log(test);ci
+						console.log(test);
 					}
 				`)),
 	}, afero.NewMemMapFs(), lib.RuntimeOptions{})

lib/options.go

@@ -244,6 +244,10 @@ type Options struct {
 	// errors about running out of file handles or sockets, or being unable to bind addresses.
 	NoVUConnectionReuse null.Bool `json:"noVUConnectionReuse" envconfig:"no_vu_connection_reuse"`
 
+	// MinIterationDuration can be used to force VUs to pause between iterations if a specific
+	// iteration is shorter than the specified value.
+	MinIterationDuration types.NullDuration `json:"minIterationDuration" envconfig:"min_iteration_duration"`
+
 	// These values are for third party collectors' benefit.
 	// Can't be set through env vars.
 	External map[string]json.RawMessage `json:"ext" ignored:"true"`
@@ -354,6 +358,9 @@ func (o Options) Apply(opts Options) Options {
 	if opts.NoVUConnectionReuse.Valid {
 		o.NoVUConnectionReuse = opts.NoVUConnectionReuse
 	}
+	if opts.MinIterationDuration.Valid {
+		o.MinIterationDuration = opts.MinIterationDuration
+	}
 	if opts.NoCookiesReset.Valid {
 		o.NoCookiesReset = opts.NoCookiesReset
 	}

release notes/upcoming.md

@@ -89,10 +89,14 @@ export default function () {
 
 Thanks to @AndriiChuzhynov for implementing this! (#766)
 
-### New option to disable the summary at the end of a test (#729)
+### New option: disable the summary at the end of a test (#729)
 
 A new option that disables the end-of-test summary has been added. That summary is often superfluous when k6 tests are run in a distributed execution mode, or when the generated metrics are piped to an external output like InfluxDB or Load Impact Insights. The option can be enabled with the `--no-summary` CLI flag or the `K6_NO_SUMMARY` environment variable. When both it and the and the `--no-thresholds` option are enabled, k6 won't store any generated metrics in-memory, making the test execution a bit more efficient.
 
+### New option: set a minimum iteration duration (#821)
+
+You can now specify the minimum amount of time a single iteration should take via the new `minIterationDuration` option. It's also configurable via the `--min-iteration-duration` CLI flag and `K6_MIN_ITERATION_DURATION` environment variable. This setting only applies for full iterations, so any interrupted iterations due to ramping down of VUs from a stage or at the end of the tests can still be shorter.
+
 ## UX
 
 * Added a warning when the maximum number of VUs is more than the total number of iterations (#802)