Revise heuristic for initial jitting. (#1949)

Increase the magic limit to 10s per #1780.

Also use the "1.5" criteria for retry so we don't have odd threshold
where initial jitting is a bit faster than IterationTime but not fast
enough to trigger the multi action engine.

Author: AndyAyersMS

src/BenchmarkDotNet/Engines/EngineFactory.cs

@@ -43,25 +43,23 @@ public IEngine CreateReadyToRun(EngineParameters engineParameters)
 
             var singleActionEngine = CreateSingleActionEngine(engineParameters);
             var singleInvocationTime = Jit(singleActionEngine, ++jitIndex, invokeCount: 1, unrollFactor: 1);
+            double timesPerIteration = engineParameters.IterationTime / singleInvocationTime; // how many times can we run given benchmark per iteration
 
-            if (singleInvocationTime > engineParameters.IterationTime && singleInvocationTime < TimeInterval.FromSeconds(1.0))
+            if ((timesPerIteration < 1.5) && (singleInvocationTime < TimeInterval.FromSeconds(10.0)))
             {
-                // if the Jitting took more than IterationTime but still less than 1s (a magic number based on observations of the reported bug)
+                // if the Jitting took more than IterationTime/1.5 but still less than 10s (a magic number based on observations of reported bugs)
                 // we call it one more time to see if Jitting itself has not dominated the first invocation
-                // if it did, it shoud NOT be a single invocation engine (see #837, #1337 and #1338)
+                // if it did, it should NOT be a single invocation engine (see #837, #1337, #1338, and #1780)
                 singleInvocationTime = Jit(singleActionEngine, ++jitIndex, invokeCount: 1, unrollFactor: 1);
+                timesPerIteration = engineParameters.IterationTime / singleInvocationTime;
             }
 
-            if (singleInvocationTime > engineParameters.IterationTime)
-                return singleActionEngine; // executing once takes longer than iteration time => long running benchmark, needs no pilot and no overhead
-
-            int defaultUnrollFactor = Job.Default.ResolveValue(RunMode.UnrollFactorCharacteristic, EngineParameters.DefaultResolver);
-
-            double timesPerIteration = engineParameters.IterationTime / singleInvocationTime; // how many times can we run given benchmark per iteration
-
-            if (timesPerIteration < 1.5) // example: IterationTime is 0.5s, but single invocation takes 0.4s => we don't want to run it twice per iteration
+            // executing once takes longer than iteration time => long running benchmark, needs no pilot and no overhead
+            // Or executing twice would put us well past the iteration time ==> needs no pilot and no overhead
+            if (timesPerIteration < 1.5)
                 return singleActionEngine;
 
+            int defaultUnrollFactor = Job.Default.ResolveValue(RunMode.UnrollFactorCharacteristic, EngineParameters.DefaultResolver);
             int roundedUpTimesPerIteration = (int)Math.Ceiling(timesPerIteration);
 
             if (roundedUpTimesPerIteration < defaultUnrollFactor) // if we run it defaultUnrollFactor times per iteration, it's going to take longer than IterationTime
@@ -129,4 +127,4 @@ private static Engine CreateEngine(EngineParameters engineParameters, Job job, A
                 engineParameters.MeasureExtraStats,
                 engineParameters.BenchmarkName);
     }
-}
+}