C#•2y ago

❔ Any reasons to use System.Memory<T> instead Array<T>

Just read docs about System.Memory<T>, tried it... I there any use cases where array can't do same work?

42 Replies

RumTeryOP•2y ago

Also. Is there any way to get memory from MemoryPool.Shared.Rent of exact same size as minimal size in method parameters?

Thinker•2y ago

Memory<T>, similarly to Span<T>, is just a view of memory, it's not an array in itself.

RumTeryOP•2y ago

Anyway, is there any use cases showing difference between their usage? The same idea is applied to array. I can use AsSpan() and modify array's data, or send it somewhere if there's no async/await/threading

cap5lut•2y ago

but with Memory<T> u can do it, as its just a struct and not a ref struct like Span<T>

RumTeryOP•2y ago

Same for array, except that it's class

cap5lut•2y ago

also u can throw a Memory<T> on native allocated memory (eg. from P/Invokes) without copying anything. with an array u would have to make a copy

RumTeryOP•2y ago

Well, this is useful case

cap5lut•2y ago

which means a heap allocation for sure, and if u just want to provide access to the first X elements with an array u would have to create a new array, so new allocations, more stuff for the GC to care about and u have to copy data. not to mention the ReadOnly variants ;p

RumTeryOP•2y ago

Ok. I got the idea And what about memory size from MemoryPool.Shared.Rent?

cap5lut•2y ago

if its too big for what ya want, just slice it shorter ;p eg
var rented = MemoryPool<byte>.Shared.Rent(1024); try { var buffer = rented[..1024]; // do stuff } finally { MemoryPool<byte>.Shared.Return(rented); }
var rented = MemoryPool<byte>.Shared.Rent(1024); try { var buffer = rented[..1024]; // do stuff } finally { MemoryPool<byte>.Shared.Return(rented); }

RumTeryOP•2y ago

Thanks a lot

cap5lut•2y ago

didnt use the memory pool yet, but i guess its similar to the array pool stuff

RumTeryOP•2y ago

There's no way to slice memoryPool owner... And no Return of rented, only Dispose 🥲 Ok. but I understood all

Aaron•2y ago

yeah it's slightly different than how they showed

using var rent = MemoryPool<byte>.Shared.Rent(1024);
var memory = rent.Memory[..1024];
// use the memory

using var rent = MemoryPool<byte>.Shared.Rent(1024);
var memory = rent.Memory[..1024];
// use the memory

cap5lut•2y ago

well, if i need to use a pool, i usually just use an array pool.

byte[] rented = MemoryPool<byte>.Shared.Rent(1024);
try {
  var buffer = rented.AsMemory()[..1024];
  // do stuff
} finally {
  ArrayPool<byte>.Shared.Return(rented);
}

byte[] rented = MemoryPool<byte>.Shared.Rent(1024);
try {
  var buffer = rented.AsMemory()[..1024];
  // do stuff
} finally {
  ArrayPool<byte>.Shared.Return(rented);
}

Aaron•2y ago

MemoryPool returns a IMemoryOwner not an array/Memory directly

cap5lut•2y ago

good to know, i should probably rewrite some of my stuff ;p

Aaron•2y ago

ArrayPool just returns a normal array though

cap5lut•2y ago

yeah, i assume the memory pool variant is better cuz less instancing oh, btw a real life example would probably be text parsing and alike

RumTeryOP•2y ago

Why Pools designed to return a bit more... awkward behaviour that forces to save size Already using it while learning OpenGL

cap5lut•2y ago

well, for array pools its probably, if they have a "free" 40 sized array and u request a 32 sized array, they wouldnt need to allocate one

RumTeryOP•2y ago

If memory is just start and size... They could just give slice of free memory

cap5lut•2y ago

maybe its because of memory alignment (i dunno too much about it, but in the end there must be a reason for it)

RumTeryOP•2y ago

especially if it can be moved by GC that size offset could be also moved and used as free in pool... Will try to find something about it

cap5lut•2y ago

maybe its also, that they simply work with "constant" chunk sizes and simply get x chunks back, so they dont have to deal with too much fragmentation

RumTeryOP•2y ago

Magic 4095 🙂

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Runtime.CompilerServices;

namespace System.Buffers
{
    internal sealed partial class ArrayMemoryPool<T> : MemoryPool<T>
    {
        public sealed override int MaxBufferSize => Array.MaxLength;

        public sealed override unsafe IMemoryOwner<T> Rent(int minimumBufferSize = -1)
        {
#pragma warning disable 8500 // sizeof of managed types
            if (minimumBufferSize == -1)
                minimumBufferSize = 1 + (4095 / sizeof(T));
            else if (((uint)minimumBufferSize) > Array.MaxLength)
                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.minimumBufferSize);
#pragma warning restore 8500

            return new ArrayMemoryPoolBuffer(minimumBufferSize);
        }

        protected sealed override void Dispose(bool disposing) { }  // ArrayMemoryPool is a shared pool so Dispose() would be a nop even if there were native resources to dispose.
    }
}

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Runtime.CompilerServices;

namespace System.Buffers
{
    internal sealed partial class ArrayMemoryPool<T> : MemoryPool<T>
    {
        public sealed override int MaxBufferSize => Array.MaxLength;

        public sealed override unsafe IMemoryOwner<T> Rent(int minimumBufferSize = -1)
        {
#pragma warning disable 8500 // sizeof of managed types
            if (minimumBufferSize == -1)
                minimumBufferSize = 1 + (4095 / sizeof(T));
            else if (((uint)minimumBufferSize) > Array.MaxLength)
                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.minimumBufferSize);
#pragma warning restore 8500

            return new ArrayMemoryPoolBuffer(minimumBufferSize);
        }

        protected sealed override void Dispose(bool disposing) { }  // ArrayMemoryPool is a shared pool so Dispose() would be a nop even if there were native resources to dispose.
    }
}

Unknown User•2y ago

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RumTeryOP•2y ago

It was Unsafe.SizeOf<T> before That this 4095 not commented any way or not set to variable/const with human-written name )

Unknown User•2y ago

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RumTeryOP•2y ago

That makes sense. But it works only minSize set to -1

Unknown User•2y ago

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RumTeryOP•2y ago

Yeah, also found code

cap5lut•2y ago

and here i am using mainly stackalloc ;p

RumTeryOP•2y ago

I think i'll just use array 🤣 and somehow convert it to memory

cap5lut•2y ago

new double[1024].AsMemory() and u r done ;p

cap5lut•2y ago

or simply use https://learn.microsoft.com/en-us/dotnet/communitytoolkit/high-performance/memoryowner

MemoryOwner - .NET Community Toolkit

A buffer type implementing IMemoryOwner that rents memory from a shared pool

RumTeryOP•2y ago

Wow, will try

RumTeryOP•2y ago

All I needed 🤣

RumTeryOP•2y ago

Thank you very much ✌️

cap5lut•2y ago

dont forget to $close the thread if ur questions are answered

MODiX•2y ago

Use the /close command to mark a forum thread as answered

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Any reasons to use System.Memory<T> instead Array<T>