Better defaults for the Pipe (#35939)

- Use a 4K buffer instead of 2K (4K is a very common buffer size and usually maps to system page size)
- Use a stack for the buffer segment pool and allow pooling more than 16 segments for large writes

src/System.IO.Pipelines/src/System/IO/Pipelines/Pipe.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Buffers;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Threading;
@@ -16,7 +17,8 @@ namespace System.IO.Pipelines
     /// </summary>
     public sealed partial class Pipe
     {
-        internal const int SegmentPoolSize = 16;
+        internal const int InitialSegmentPoolSize = 16; // 65K
+        internal const int MaxPoolSize = 256; // 1MB
 
         private static readonly Action<object> s_signalReaderAwaitable = state => ((Pipe)state).ReaderCancellationRequested();
         private static readonly Action<object> s_signalWriterAwaitable = state => ((Pipe)state).WriterCancellationRequested();
@@ -42,7 +44,7 @@ public sealed partial class Pipe
         private readonly PipeScheduler _readerScheduler;
         private readonly PipeScheduler _writerScheduler;
 
-        private readonly BufferSegment[] _bufferSegmentPool;
+        private readonly Stack<BufferSegment> _bufferSegmentPool;
 
         private readonly DefaultPipeReader _reader;
         private readonly DefaultPipeWriter _writer;
@@ -52,8 +54,6 @@ public sealed partial class Pipe
         private long _length;
         private long _currentWriteLength;
 
-        private int _pooledSegmentCount;
-
         private PipeAwaitable _readerAwaitable;
         private PipeAwaitable _writerAwaitable;
 
@@ -101,7 +101,7 @@ public Pipe(PipeOptions options)
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.options);
             }
 
-            _bufferSegmentPool = new BufferSegment[SegmentPoolSize];
+            _bufferSegmentPool = new Stack<BufferSegment>(InitialSegmentPoolSize);
 
             _operationState = default;
             _readerCompletion = default;
@@ -253,10 +253,9 @@ private int GetSegmentSize(int sizeHint, int maxBufferSize = int.MaxValue)
 
         private BufferSegment CreateSegmentUnsynchronized()
         {
-            if (_pooledSegmentCount > 0)
+            if (_bufferSegmentPool.Count > 0)
             {
-                _pooledSegmentCount--;
-                return _bufferSegmentPool[_pooledSegmentCount];
+                return _bufferSegmentPool.Pop();
             }
 
             return new BufferSegment();
@@ -269,10 +268,9 @@ private void ReturnSegmentUnsynchronized(BufferSegment segment)
             Debug.Assert(segment != _writingHead, "Returning _writingHead segment that's in use!");
             Debug.Assert(segment != _lastExamined, "Returning _lastExamined segment that's in use!");
 
-            if (_pooledSegmentCount < _bufferSegmentPool.Length)
+            if (_bufferSegmentPool.Count < MaxPoolSize)
             {
-                _bufferSegmentPool[_pooledSegmentCount] = segment;
-                _pooledSegmentCount++;
+                _bufferSegmentPool.Push(segment);
             }
         }
 

src/System.IO.Pipelines/src/System/IO/Pipelines/PipeOptions.cs

@@ -12,11 +12,11 @@ namespace System.IO.Pipelines
     /// </summary>
     public class PipeOptions
     {
-        private const int DefaultMinimumSegmentSize = 2048;
+        private const int DefaultMinimumSegmentSize = 4096;
 
-        private const int DefaultResumeWriterThreshold = DefaultMinimumSegmentSize * Pipe.SegmentPoolSize / 2;
+        private const int DefaultResumeWriterThreshold = DefaultMinimumSegmentSize * Pipe.InitialSegmentPoolSize / 2;
 
-        private const int DefaultPauseWriterThreshold = DefaultMinimumSegmentSize * Pipe.SegmentPoolSize;
+        private const int DefaultPauseWriterThreshold = DefaultMinimumSegmentSize * Pipe.InitialSegmentPoolSize;
 
         /// <summary>
         /// Default instance of <see cref="PipeOptions"/>

src/System.IO.Pipelines/tests/PipeOptionsTests.cs

@@ -12,13 +12,13 @@ public class PipeOptionsTests
         [Fact]
         public void DefaultPauseWriterThresholdIsSet()
         {
-            Assert.Equal(32768, PipeOptions.Default.PauseWriterThreshold);
+            Assert.Equal(65536, PipeOptions.Default.PauseWriterThreshold);
         }
 
         [Fact]
         public void DefaultResumeWriterThresholdIsSet()
         {
-            Assert.Equal(16384, PipeOptions.Default.ResumeWriterThreshold);
+            Assert.Equal(32768, PipeOptions.Default.ResumeWriterThreshold);
         }
 
         [Fact]

src/System.IO.Pipelines/tests/PipePoolTests.cs

@@ -206,7 +206,12 @@ public void ReturnsWriteHeadOnComplete()
         public void ReturnsWriteHeadWhenRequestingLargerBlock()
         {
             var pool = new DisposeTrackingBufferPool();
-            var pipe = new Pipe(CreatePipeWithInlineSchedulers(pool));
+            var options = new PipeOptions(pool,
+                readerScheduler: PipeScheduler.Inline,
+                writerScheduler: PipeScheduler.Inline,
+                minimumSegmentSize: 2048);
+
+            var pipe = new Pipe(options);
             pipe.Writer.GetMemory(512);
             pipe.Writer.GetMemory(4096);