Drain threads for memory cache hit rate analysis (#553)

* drain threads

* faster

* fix title

---------

Author: bitfaster

BitFaster.Caching.HitRateAnalysis/Arc/Runner.cs

@@ -31,7 +31,7 @@ public async Task Run()
 
             this.config.Analysis.WriteToConsole();
             Analysis<long>.WriteToFile(this.config.Name, this.config.Analysis);
-            Analysis<long>.Plot(this.config.Title, this.config.Name, this.config.Analysis);
+            Analysis<long>.Plot(this.config.Name, this.config.Title, this.config.Analysis);
         }
 
         private int AnalyzeSmall()

BitFaster.Caching.HitRateAnalysis/MemoryCacheAdaptor.cs

@@ -63,6 +63,7 @@ public V GetOrAdd(K key, Func<K, V> valueFactory)
                 entry.SetSize(1);
 
                 this.metrics.requestMissCount++;
+                ThreadPoolInspector.WaitForEmpty();
             }
             else
             {

BitFaster.Caching.HitRateAnalysis/ThreadPoolInspector.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Threading;
+
+namespace BitFaster.Caching.HitRateAnalysis
+{
+    internal class ThreadPoolInspector
+    {
+        public static void WaitForEmpty()
+        {
+            while (ThreadPool.PendingWorkItemCount > 0)
+            {
+                Thread.Yield();
+
+                // This is very hacky, but by experimentation 300 Sleep(0) consistently takes longer 
+                // than cache maintenance giving stable results with around 25% run time penalty.
+                // Sleep(1) makes the test take 50x longer.
+                if (ThreadPool.PendingWorkItemCount == 0)
+                {
+                    for (int i = 0; i < 300; i++)
+                    {
+                        Thread.Sleep(0);
+                    }
+                }
+            }
+        }
+    }
+}