Added SegmentedPool
This commit is contained in:
Jim
0xJ1M
445
UnmangedMMU/SegmentedPool.cs
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445
UnmangedMMU/SegmentedPool.cs
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@@ -0,0 +1,445 @@
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namespace UnmanagedMMU
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{
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using System;
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using System.Collections.Generic;
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using System.Runtime.InteropServices;
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using System.Threading;
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using UnmanagedMMU.Allocators;
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/// <summary>
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/// Implementation of segmented Bump-Allocator.
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///
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/// This implementation manages fixed-sized unmanaged memory segments.
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/// This ensures allocations are fast and contiguous within a segment.
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/// Once a segment is full, a new one is automatically allocated
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/// </summary>
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public unsafe sealed class SegmentedPool : IDisposable
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{
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/// <summary>
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/// The default size for a <see cref="Segment"/>
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/// </summary>
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private const nuint _defaultSegmentSize = 4194304; // 4 MiB
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/// <summary>
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/// The size that a new <see cref="Segment"/> should be
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/// </summary>
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private nuint _currentSegmentSize;
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/// <summary>
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/// Queue of free segments
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/// </summary>
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private readonly Stack<IntPtr> _freeSegments = new();
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/// <summary>
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/// List of active memory segments
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/// </summary>
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private readonly List<IntPtr> _activeSegments = [];
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/// <summary>
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/// Tracks the total amount of allocated bytes
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/// </summary>
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private nuint _totalAllocated = 0;
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/// <summary>
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/// Tracks the total amount of allocated bytes current in use
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/// </summary>
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private nuint _totalUsed = 0;
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/// <summary>
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/// Internal lock, ensures thread safety while maintaining a simple interface
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/// </summary>
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private readonly Lock _lock = new();
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/// <summary>
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/// Indicates whether the <see cref="SegmentedPool"/> has been disposed.
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/// </summary>
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private volatile bool _disposed;
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/// <summary>
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/// Pointer to the currently in use <see cref="Segment"/>
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/// </summary>
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private Segment* _current;
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private readonly IUnmanagedAllocator _allocator;
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/// <summary>
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/// Represents a memory segment in the <see cref="SegmentedPool"/>.
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/// </summary>
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/// <remarks>
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/// <list type="bullet">
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/// <item><description><see cref="Ptr"/> Pointer to the start of the unmanaged memory block</description></item>
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/// <item><description><see cref="Offset"/> indicates the current allocation position within the segment. Each new allocation advances this offset by the number of bytes allocated.</description></item>
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/// <item><description><see cref="Size"/> is the total size, in bytes, of the segment.</description></item>
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/// <item><description>Segments are managed internally by the <see cref="SegmentedPool"/> and should not be modified directly outside of the pool.</description></item>
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/// <item><description>A <see cref="Segment"/> is a contiguous block of unmanaged memory from which allocations are served sequentially via bump allocation.</description></item>
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/// </list>
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/// </remarks>
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private struct Segment
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{
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/// <summary>
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/// Pointer to the start of the unmanaged memory block.
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/// </summary>
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public byte* Ptr;
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/// <summary>
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/// The current offset into the <see cref="Segment"/> where the next allocation will occur.
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/// </summary>
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public nuint Offset;
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/// <summary>
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/// Total size of the <see cref="Segment"/> in bytes.
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/// </summary>
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public nuint Size;
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}
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/// <summary>
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/// Initializes a new <see cref="SegmentedPool"/> with the specified default <paramref name="segmentSize"/> and <paramref name="initialSegments"/> count
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/// </summary>
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/// <param name="segmentSize">Size of each segment in bytes (default 4 MiB)</param>
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/// <param name="initialSegments">Number of segments to pre-allocate to the pool</param>
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/// <exception cref="ArgumentException">
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/// Thrown if <paramref name="segmentSize"/> is zero, or if <paramref name="initialSegments"/> is less than 1.
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/// </exception>
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public SegmentedPool(nuint segmentSize = _defaultSegmentSize, int initialSegments = 4)
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: this(segmentSize, initialSegments, new DefaultUnmanagedAllocator())
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{
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}
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/// <summary>
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/// Initializes a new <see cref="SegmentedPool"/> with the specified default <paramref name="segmentSize"/> and <paramref name="initialSegments"/> count
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/// </summary>
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/// <param name="segmentSize">Size of each segment in bytes (default 4 MiB)</param>
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/// <param name="initialSegments">Number of segments to pre-allocate to the pool</param>
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/// <param name="allocator">IUnmanagedAllocator instance that implements the allocator</param>
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/// <exception cref="ArgumentException">
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/// Thrown if <paramref name="segmentSize"/> is zero, or if <paramref name="initialSegments"/> is less than 1.
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/// </exception>
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internal SegmentedPool(nuint segmentSize, int initialSegments, IUnmanagedAllocator allocator)
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{
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if (segmentSize == 0)
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{
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throw new ArgumentException("Segment size must be greater than zero.", nameof(segmentSize));
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}
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if (initialSegments < 1)
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{
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throw new ArgumentException("Initial segments count must be at least 1.", nameof(initialSegments));
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}
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_allocator = allocator;
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_currentSegmentSize = segmentSize;
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// Pre-allocate segments
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for (int i = 0; i < initialSegments; i++)
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{
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_freeSegments.Push((IntPtr)AllocateNewSegment(_currentSegmentSize));
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}
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_current = (Segment*)_freeSegments.Pop();
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_activeSegments.Add((IntPtr)_current);
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}
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/// <summary>
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/// Gets the number of free segments available for use in the pool.
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/// </summary>
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/// <returns>The number of free segments.</returns>
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public int FreeSegmentCount
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{
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get { return _freeSegments.Count; }
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}
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/// <summary>
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/// Gets the number of segments currently in use in the pool.
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/// </summary>
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/// <returns>The number of currently active Segments.</returns>
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public int ActiveSegmentCount
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{
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get { return _activeSegments.Count; }
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}
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/// <summary>
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/// Returns the total number of bytes that have currently been allocated.
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/// </summary>
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/// <returns>The total number of bytes that have been allocated.</returns>
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public nuint TotalAllocatedBytes
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{
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get { return _totalAllocated; }
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}
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/// <summary>
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/// Gets the total number of bytes currently in use across all active segments.
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/// </summary>
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/// <returns>The total number of bytes that are in use.</returns>
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public nuint TotalUsedBytes
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{
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get { return _totalUsed; }
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}
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/// <summary>
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/// Gets the size, in bytes, used when allocating new <see cref="Segment"/> instances.
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/// </summary>
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/// <remarks>
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/// This reflects the most recently configured size and affects only future <see cref="Segment"/> allocations.
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/// </remarks>
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public nuint CurrentSegmentSize
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{
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get { return _currentSegmentSize; }
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}
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/// <summary>
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/// Gets a value indicating whether this <see cref="SegmentedPool"/> instance has been disposed.
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/// </summary>
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/// <returns>True, if the current <see cref="SegmentedPool"/> instance has been disposed of, false otherwise.</returns>
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/// <remarks>
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/// Once disposed, further calls to allocation or reset methods will throw <see cref="ObjectDisposedException"/>.
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/// </remarks>
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public bool IsDisposed
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{
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get { return _disposed; }
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}
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/// <summary>
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/// Sets the current <see cref="Segment"/> size used for subsequent allocations.
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/// </summary>
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/// <param name="newSize">
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/// The new segment size, in bytes, that will be used when allocating future <see cref="Segment"/> instances.
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/// Must be greater than zero.
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/// </param>
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/// <remarks>
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/// This does not affect any <see cref="Segment"/> that have already been allocated or are currently active.
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/// Only new <see cref="Segment"/> created after calling this method will use the updated size.
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/// </remarks>
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/// <exception cref="ArgumentOutOfRangeException">
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/// Thrown if <paramref name="newSize"/> is zero.
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/// </exception>
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public void SetSegmentSize(nuint newSize)
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{
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ThrowIfDisposed();
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if (newSize == 0)
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{
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throw new ArgumentOutOfRangeException(nameof(newSize), "Segment size must be greater than zero.");
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}
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_currentSegmentSize = newSize;
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}
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/// <summary>
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/// Resets the current <see cref="Segment"/> size back to the default 4 MiB
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/// </summary>
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/// <remarks>
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/// Future <see cref="Segment"/> allocations will revert to using this default size
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/// Active and free <see cref="Segment"/> are not modified.
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/// </remarks>
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public void ResetSegmentSize()
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{
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ThrowIfDisposed();
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_currentSegmentSize = _defaultSegmentSize;
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}
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/// <summary>
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/// Allocates a span of unmanaged memory of size <paramref name="count"/> for elements of type <typeparamref name="T"/>.
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/// </summary>
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/// <typeparam name="T">The unmanaged value type to store in the allocated memory. Must be a struct or primitive type.</typeparam>
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/// <param name="count">The number of elements of type <typeparamref name="T"/> to allocate.</param>
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/// <returns>A <see cref="Span{T}"/> representing the allocated memory. The span is valid until the <see cref="SegmentedPool"/> is reset or disposed.</returns>
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/// <remarks>
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/// <list type="bullet">
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/// <item><description>This allocation is performed in unmanaged memory and bypasses the .NET garbage collector.</description></item>
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/// <item><description>Accessing the memory after <see cref="Reset"/> or <see cref="Dispose"/> has been called is undefined behavior and may lead to crashes.</description></item>
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/// </list>
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/// </remarks>
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/// <exception cref="ArgumentOutOfRangeException">
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/// Thrown if <paramref name="count"/> is less than or equal to zero.
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/// </exception>
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/// <exception cref="OverflowException">
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/// Thrown if the total allocation size (count * sizeof(T)) exceeds the maximum allowable size.
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/// </exception>
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public Span<T> Allocate<T>(int count) where T : unmanaged
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{
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ThrowIfDisposed();
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if (count <= 0)
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{
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throw new ArgumentOutOfRangeException(nameof(count), "Allocation count must be greater than zero.");
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}
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if ((nuint)count > nuint.MaxValue / (nuint)(sizeof(T)))
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{
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throw new OverflowException($"Requested allocation of {count} elements of type {typeof(T)} exceeds allowable maximum memory size.");
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}
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nuint bytes = (nuint)(count * sizeof(T));
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lock (_lock)
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{
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// Enough space in current segment?
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if (_current->Offset + bytes > _current->Size)
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SwitchSegment(bytes);
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T* ptr = (T*)(_current->Ptr + _current->Offset);
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_current->Offset += bytes;
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_totalUsed += bytes;
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return new Span<T>(ptr, count);
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}
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}
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/// <summary>
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/// Switches to a new <see cref="Segment"/> when the current <see cref="Segment"/> is full.
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/// </summary>
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/// <param name="requiredBytes">The number of bytes required for the upcoming allocation. If the current <see cref="Segment"/> does not have enough free space, a new <see cref="Segment"/> will be used.</param>
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private void SwitchSegment(nuint requiredBytes)
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{
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Segment* segment;
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// Allocate fresh Segment if needed
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if (_freeSegments.Count == 0 || requiredBytes > _currentSegmentSize)
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{
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segment = AllocateNewSegment(requiredBytes > _currentSegmentSize ? requiredBytes : _currentSegmentSize);
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}
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else
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{
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segment = (Segment*)_freeSegments.Pop();
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segment->Offset = 0;
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}
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_activeSegments.Add((IntPtr)segment);
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_current = segment;
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}
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/// <summary>
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/// Allocates a new <see cref="Segment"/>
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/// </summary>
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/// <param name="size">
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/// Optional size, in bytes, for the new <see cref="Segment"/>.
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/// If <c>null</c>, the default <see cref="Segment"/> size (<see cref="_defaultSegmentSize"/>) is used.
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/// </param>
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/// <returns>A pointer to the newly allocated <see cref="Segment"/></returns>
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private Segment* AllocateNewSegment(nuint size)
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{
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byte* ptr = (byte*)_allocator.Alloc(size);
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Segment* segment = (Segment*)_allocator.Alloc((nuint)sizeof(Segment));
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segment->Ptr = ptr;
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segment->Offset = 0;
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segment->Size = size;
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_totalAllocated += size;
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return segment;
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}
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/// <summary>
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/// Frees unused <see cref="Segment"/> in the free pool, reducing unmanaged memory usage.
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/// </summary>
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/// <param name="minFreeSegments"> Minimum number of free segments to retain for future allocations. Defaults to <c>16</c>. </param>
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/// <remarks>
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/// Segments beyond the retained count will have their unmanaged memory released back to the system.
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/// </remarks>
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/// <exception cref="ArgumentOutOfRangeException">
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/// Thrown if <paramref name="minFreeSegments"/> is negative.
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/// </exception>
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public void Trim(int minFreeSegments = 16)
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{
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ThrowIfDisposed();
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ArgumentOutOfRangeException.ThrowIfNegative(minFreeSegments, nameof(minFreeSegments));
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lock (_lock)
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{
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while (_freeSegments.Count > minFreeSegments)
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{
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var ip = _freeSegments.Pop();
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Segment* segment = (Segment*)ip;
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// Free the unmanaged memory
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_allocator.Free(segment->Ptr);
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_totalAllocated -= segment->Size;
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_allocator.Free(segment);
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}
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}
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}
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/// <summary>
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/// Resets the allocator, returning all active segments to the free pool.
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/// </summary>
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/// <param name="trim">If true, trims the <see cref="Segment"/> in the free pool. <see cref="Trim"/></param>
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public void Reset(bool trim = false)
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{
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ThrowIfDisposed();
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lock (_lock)
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{
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foreach (var ip in _activeSegments)
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{
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Segment* segment = (Segment*)ip;
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segment->Offset = 0;
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_freeSegments.Push(ip);
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}
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_activeSegments.Clear();
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_totalUsed = 0;
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if (_freeSegments.Count > 0)
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{
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_current = (Segment*)_freeSegments.Pop();
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_activeSegments.Add((IntPtr)_current);
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}
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else
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{
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// This should not be hit in normal circumstances as we always have _current
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_current = AllocateNewSegment(_currentSegmentSize);
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_activeSegments.Add((IntPtr)_current);
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}
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// Optionally trim excess free segments after reset
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if (trim)
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{
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Trim();
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}
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}
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}
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/// <summary>
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/// Releases all unmanaged memory allocated by the <see cref="SegmentedPool"/> and clears internal state.
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/// After calling this method, the pool can no longer be used for allocations.
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/// </summary>
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/// <remarks>
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/// After disposal, all allocations become invalid and any further operations will throw <see cref="ObjectDisposedException"/>.
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/// This method is thread-safe and may be called multiple times safely.
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/// </remarks>
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public void Dispose()
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{
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lock (_lock)
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{
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if (_disposed)
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{
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return;
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}
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// Free active pages
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foreach (var ip in _activeSegments)
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{
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Segment* segment = (Segment*)ip;
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_allocator.Free(segment->Ptr);
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_allocator.Free(segment);
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}
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// Free free pages
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foreach (var ip in _freeSegments)
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{
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Segment* segment = (Segment*)ip;
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_allocator.Free(segment->Ptr);
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_allocator.Free(segment);
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}
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_activeSegments.Clear();
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_freeSegments.Clear();
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_current = null;
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_totalAllocated = 0;
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_totalUsed = 0;
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_disposed = true;
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}
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}
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/// <summary>
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/// Throws an <see cref="ObjectDisposedException"/> if the <see cref="SegmentedPool"/> has already been disposed.
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/// </summary>
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/// <exception cref="ObjectDisposedException">
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/// Thrown when this instance is no longer valid for use.
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/// </exception>
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private void ThrowIfDisposed()
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{
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ObjectDisposedException.ThrowIf(_disposed, this);
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}
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}
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}
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