Added request latency probing service to monitor thread pool starvation.

Author: rhamlett_microsoft

src/PerfProblemSimulator/Controllers/HealthController.cs

@@ -100,6 +100,34 @@ public IActionResult GetStatus()
                 .ToList()
         });
     }
+
+    /// <summary>
+    /// Lightweight probe endpoint for latency measurement.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <strong>Educational Note:</strong> This endpoint is designed for measuring
+    /// request processing latency. It does minimal work - just returns a timestamp.
+    /// During thread pool starvation, even this simple endpoint will show increased
+    /// latency because the request must wait for a thread pool thread to process it.
+    /// </para>
+    /// <para>
+    /// Compare the response time of this endpoint before and during thread pool
+    /// starvation to see the dramatic impact on user-perceived performance.
+    /// </para>
+    /// </remarks>
+    /// <response code="200">Returns probe timestamp for latency calculation.</response>
+    [HttpGet("probe")]
+    [ProducesResponseType(typeof(ProbeResponse), StatusCodes.Status200OK)]
+    public IActionResult Probe()
+    {
+        return Ok(new ProbeResponse
+        {
+            ServerTimestamp = DateTimeOffset.UtcNow,
+            ThreadPoolThreads = ThreadPool.ThreadCount,
+            PendingWorkItems = ThreadPool.PendingWorkItemCount
+        });
+    }
 }
 
 /// <summary>
@@ -159,3 +187,31 @@ public class ActiveSimulationSummary
     /// </summary>
     public int RunningDurationSeconds { get; init; }
 }
+
+/// <summary>
+/// Response from the latency probe endpoint.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Educational Note:</strong> This response includes thread pool information
+/// to help correlate latency with thread pool state. During starvation, you'll see
+/// high pending work items and latency increasing together.
+/// </para>
+/// </remarks>
+public class ProbeResponse
+{
+    /// <summary>
+    /// Server timestamp when the probe was processed.
+    /// </summary>
+    public DateTimeOffset ServerTimestamp { get; init; }
+
+    /// <summary>
+    /// Current number of thread pool threads.
+    /// </summary>
+    public int ThreadPoolThreads { get; init; }
+
+    /// <summary>
+    /// Number of work items waiting in the thread pool queue.
+    /// </summary>
+    public long PendingWorkItems { get; init; }
+}

src/PerfProblemSimulator/Hubs/IMetricsClient.cs

@@ -1,4 +1,5 @@
 using PerfProblemSimulator.Models;
+using PerfProblemSimulator.Services;
 
 namespace PerfProblemSimulator.Hubs;
 
@@ -41,4 +42,17 @@ public interface IMetricsClient
     /// <param name="simulationType">Type of simulation that completed.</param>
     /// <param name="simulationId">ID of the simulation.</param>
     Task SimulationCompleted(string simulationType, Guid simulationId);
+
+    /// <summary>
+    /// Receives a latency measurement from the server-side probe.
+    /// </summary>
+    /// <param name="measurement">The latency measurement data.</param>
+    /// <remarks>
+    /// <para>
+    /// <strong>Educational Note:</strong> This measurement shows real request processing
+    /// latency. Compare baseline latency (~5-20ms) with latency during thread pool
+    /// starvation (can exceed 30 seconds!) to see the impact of blocking threads.
+    /// </para>
+    /// </remarks>
+    Task ReceiveLatency(LatencyMeasurement measurement);
 }

src/PerfProblemSimulator/Program.cs

@@ -114,6 +114,13 @@
 // This bridges the MetricsCollector (which fires events) with SignalR (which pushes to clients).
 builder.Services.AddHostedService<MetricsBroadcastService>();
 
+// LatencyProbeService - Hosted service that measures request latency on a dedicated thread
+// Educational Note: This service demonstrates how thread pool starvation affects request
+// processing time. It runs on a dedicated thread (not the thread pool) to ensure it can
+// always measure latency, even during severe starvation conditions.
+builder.Services.AddHttpClient("LatencyProbe");
+builder.Services.AddHostedService<LatencyProbeService>();
+
 // -----------------------------------------------------------------------------
 // CORS Configuration
 // -----------------------------------------------------------------------------