Increase the default number of IO queues on larger machines

The number of IO queues is currently limited to 16. On machines with more processors, increasing the number of IO queues appears to improve throughput on some benchmarks.

Author: Koundinya Veluri

src/Servers/Kestrel/Transport.Sockets/src/Internal/IOQueue.cs

@@ -10,6 +10,8 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.Internal;
 
 internal sealed class IOQueue : PipeScheduler, IThreadPoolWorkItem
 {
+    public static readonly int DefaultCount = DetermineDefaultCount();
+
     private readonly ConcurrentQueue<Work> _workItems = new ConcurrentQueue<Work>();
     private int _doingWork;
 
@@ -73,4 +75,19 @@ public Work(Action<object?> callback, object? state)
             State = state;
         }
     }
+
+    private static int DetermineDefaultCount()
+    {
+        // Since each IOQueue schedules one work item to process its work, the number of IOQueues determines the maximum
+        // parallelism of processing work queued to IOQueues. The default number below is based on the processor count and tries
+        // to use a high-enough number for that to not be a significant limiting factor for throughput.
+
+        int processorCount = Environment.ProcessorCount;
+        if (OperatingSystem.IsWindows() || processorCount <= 32)
+        {
+            return Math.Min(processorCount, 16);
+        }
+
+        return (processorCount + 1) / 2;
+    }
 }

src/Servers/Kestrel/Transport.Sockets/src/SocketConnectionFactoryOptions.cs

@@ -33,9 +33,9 @@ internal SocketConnectionFactoryOptions(SocketTransportOptions transportOptions)
     /// The number of I/O queues used to process requests. Set to 0 to directly schedule I/O to the ThreadPool.
     /// </summary>
     /// <remarks>
-    /// Defaults to <see cref="Environment.ProcessorCount" /> rounded down and clamped between 1 and 16.
+    /// Defaults to a value based on and limited to <see cref="Environment.ProcessorCount" />.
     /// </remarks>
-    public int IOQueueCount { get; set; } = Math.Min(Environment.ProcessorCount, 16);
+    public int IOQueueCount { get; set; } = Internal.IOQueue.DefaultCount;
 
     /// <summary>
     /// Wait until there is data available to allocate a buffer. Setting this to false can increase throughput at the cost of increased memory usage.

src/Servers/Kestrel/Transport.Sockets/src/SocketTransportOptions.cs

@@ -28,9 +28,9 @@ static SocketTransportOptions()
     /// The number of I/O queues used to process requests. Set to 0 to directly schedule I/O to the ThreadPool.
     /// </summary>
     /// <remarks>
-    /// Defaults to <see cref="Environment.ProcessorCount" /> rounded down and clamped between 1 and 16.
+    /// Defaults to a value based on and limited to <see cref="Environment.ProcessorCount" />.
     /// </remarks>
-    public int IOQueueCount { get; set; } = Math.Min(Environment.ProcessorCount, 16);
+    public int IOQueueCount { get; set; } = Internal.IOQueue.DefaultCount;
 
     /// <summary>
     /// Wait until there is data available to allocate a buffer. Setting this to false can increase throughput at the cost of increased memory usage.