docs: mention Jitting phase in How it works (#2942)

* docs: mention Jitting phase in How it works

* docs: add JittingStage to pseudocode

* Update docs/articles/guides/how-it-works.md

---------

Co-authored-by: Tim Cassell <[email protected]>

Author: fuguiKz

docs/articles/guides/how-it-works.md

@@ -6,6 +6,7 @@ BenchmarkDotNet follows the following steps to run your benchmarks:
 2. Next, we take each method/job/params combination and try to measure its performance by launching benchmark process several times (`LaunchCount`).
 3. An invocation of the workload method is an *operation*. A bunch of operation is an *iteration*. If you have an `IterationSetup` method, it will be invoked before each iteration, 
 but not between operations. We have the following type of iterations:
+    * `Jitting`: The overhead/workload methods are invoked to ensure they are JIT-compiled (and on tiered runtimes, to promote them when possible). These iterations are not used for measurements.
     * `Pilot`: The best operation count will be chosen.
     * `OverheadWarmup`, `OverheadWorkload`: BenchmarkDotNet overhead will be evaluated.
     * `ActualWarmup`: Warmup of the workload method.
@@ -35,6 +36,7 @@ IEnumerable<Results> Run(Benchmark benchmark)
 Result ActualRun(Method method, Job job)
 {
     GlobalSetup();
+    JittingStage(method); // triggers JIT compilation (and tiering if enabled) before Pilot/Warmup
 
     int unrollFactor = job.Run.UnrollFactor; // 16 by default
 
@@ -54,6 +56,19 @@ Result ActualRun(Method method, Job job)
     return (result - Median(overhead), gcStats);
 }
 
+void JittingStage(Method method)
+{
+    RunIteration(method, invokeCount: 1, unrollFactor: 1);
+    if (JitInfo.IsTiered)
+    {
+        for (int i = 0; i < JitInfo.MaxTierPromotions; i++)
+        {
+            RunIteration(method, invokeCount: JitInfo.TieredCallCountThreshold, unrollFactor: 1);
+            Thread.Sleep(250);
+        }
+    }
+}
+
 long Pilot(Method method, int unrollFactor)
 {
     // invokeCount is the equivalent of InnerIterationCount from xunit-performance