RPS mode without RpsCooldownSeconds offset with fixed tests

Signed-off-by: Lazar Cvetković <[email protected]>

docs/configuration.md

@@ -14,7 +14,7 @@
 | AsyncWaitToCollectMin [^6]   | int       | >= 0                                                                | 0                   | Time after experiment ends after which to collect invocation results                 |  
 | RpsTarget                    | int       | >= 0                                                                | 0                   | Number of requests per second to issue                                               | 
 | RpsColdStartRatioPercentage  | int       | >= 0 && <= 100                                                      | 0                   | Percentage of cold starts out of specified RPS                                       | 
-| RpsCooldownSeconds           | int       | > 0                                                                 | 0                   | The time it takes for the autoscaler to downscale function (higher for higher RPS)   |
+| RpsCooldownSeconds [^7]      | int       | > 0                                                                 | 0                   | The time it takes for the autoscaler to downscale function (higher for higher RPS)   |
 | RpsImage                     | string    | N/A                                                                 | N/A                 | Function image to use for RPS experiments                                            |
 | RpsRuntimeMs                 | int       | >=0                                                                 | 0                   | Requested execution time                                                             |
 | RpsMemoryMB                  | int       | >=0                                                                 | 0                   | Requested memory                                                                     |
@@ -55,6 +55,13 @@ Lambda; https://aws.amazon.com/about-aws/whats-new/2018/10/aws-lambda-supports-f
 
 [^6]: Dirigent specific
 
+[^7] Because Knative's minimum autoscaling stable window is 6s, the minimum keep-alive for a function is 6s. This means
+that we need multiple functions to achieve RPS=1, each scaling up/and down with a 1-second delay from each other. In RPS
+mode, the number of functions for the cold start experiment is determined by the `RpsCooldownSeconds` parameter, which
+is the minimum keep-alive. Due to the implementation complexity, the cold start experiment sleeps for the first
+`RpsCooldownSeconds` seconds. In the results, the user should discard the first and the last `RpsCooldownSeconds` of the
+results, since the RPS at those points is lower than the requested one.
+
 ---
 
 InVitro can cause failure on cluster manager components. To do so, please configure the `cmd/failure.json`. Make sure

pkg/generator/rps.go

@@ -34,6 +34,11 @@ func generateFunctionByRPS(experimentDuration int, rpsTarget float64) (common.IA
 		}
 	}
 
+	// make the first invocation be fired right away
+	if len(iatResult) > 0 {
+		iatResult[0] = 0
+	}
+
 	return iatResult, countResult
 }
 
@@ -52,6 +57,13 @@ func GenerateWarmStartFunction(experimentDuration int, rpsTarget float64) (commo
 	return generateFunctionByRPS(experimentDuration, rpsTarget)
 }
 
+// GenerateColdStartFunctions Because Knative's minimum autoscaling stable window is 6s, the minimum keep-alive for a
+// function is 6s. This means that we need multiple functions to achieve RPS=1, each scaling up/and down with a 1-second
+// delay from each other. In RPS mode, the number of functions for the cold start experiment is determined by the
+// RpsCooldownSeconds parameter, which is the minimum keep-alive. This produces the IAT array as above. Due to the
+// implementation complexity, the cold start experiment sleeps for the first RpsCooldownSeconds seconds. In the results,
+// the user should discard the first and the last RpsCooldownSeconds of the results, since the RPS at those points is
+// lower than the requested one.
 func GenerateColdStartFunctions(experimentDuration int, rpsTarget float64, cooldownSeconds int) ([]common.IATArray, [][]int) {
 	iat := 1000000.0 / float64(rpsTarget) // ms
 	totalFunctions := int(math.Ceil(rpsTarget * float64(cooldownSeconds)))