- Add a minimum size threshold for array pool usage

- Add a null memory manager that doesn't do any actual memory management

src/ImageSharp/Memory/ArrayPoolMemoryManager.cs

@@ -1,4 +1,5 @@
 using System.Buffers;
+using System.Runtime.InteropServices;
 
 namespace SixLabors.ImageSharp.Memory
 {
@@ -7,9 +8,30 @@ namespace SixLabors.ImageSharp.Memory
     /// </summary>
     public class ArrayPoolMemoryManager : MemoryManager
     {
+        private readonly int minSizeBytes;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ArrayPoolMemoryManager"/> class.
+        /// By passing an integer greater than 0 as <paramref name="minSizeBytes"/>, a
+        /// minimum threshold for pooled allocations is set. Any allocation requests that
+        /// would require less size than the threshold will not be managed within the array pool.
+        /// </summary>
+        /// <param name="minSizeBytes">
+        /// Minimum size, in bytes, before an array pool is used to satisfy the request.
+        /// </param>
+        public ArrayPoolMemoryManager(int minSizeBytes = 0)
+        {
+            this.minSizeBytes = minSizeBytes;
+        }
+
         /// <inheritdoc />
         internal override Buffer<T> Allocate<T>(int size, bool clear = false)
         {
+            if (this.minSizeBytes > 0 && size < this.minSizeBytes * SizeHelper<T>.Size)
+            {
+                return new Buffer<T>(new T[size], size);
+            }
+
             var buffer = new Buffer<T>(PixelDataPool<T>.Rent(size), size, this);
             if (clear)
             {
@@ -24,5 +46,19 @@ namespace SixLabors.ImageSharp.Memory
         {
             PixelDataPool<T>.Return(buffer.Array);
         }
+
+        internal static class SizeHelper<T>
+        {
+            static SizeHelper()
+            {
+                #if NETSTANDARD1_1
+                Size = Marshal.SizeOf(typeof(T));
+                #else
+                Size = Marshal.SizeOf<T>();
+                #endif
+            }
+
+            public static int Size { get; }
+        }
     }
 }

src/ImageSharp/Memory/MemoryManager.cs

@@ -14,7 +14,7 @@ namespace SixLabors.ImageSharp.Memory
         /// <summary>
         /// Gets or sets the <see cref="MemoryManager"/> that is currently in use.
         /// </summary>
-        public static MemoryManager Current { get; set; } = new ArrayPoolMemoryManager();
+        public static MemoryManager Current { get; set; } = new ArrayPoolMemoryManager(1024 * 80);
 
         /// <summary>
         /// Allocates a <see cref="Buffer{T}"/> of size <paramref name="size"/>, optionally

src/ImageSharp/Memory/NullMemoryManager.cs

@@ -0,0 +1,19 @@
+namespace SixLabors.ImageSharp.Memory
+{
+    /// <summary>
+    /// Implements <see cref="MemoryManager"/> by allocating new buffers on every call.
+    /// </summary>
+    public class NullMemoryManager : MemoryManager
+    {
+        /// <inheritdoc />
+        internal override Buffer<T> Allocate<T>(int size, bool clear = false)
+        {
+            return new Buffer<T>(new T[size], size);
+        }
+
+        /// <inheritdoc />
+        internal override void Release<T>(Buffer<T> buffer)
+        {
+        }
+    }
+}