Tutorial Renting Buffers with ArrayPool<T>.Shared: Common Gotchas, Sizing & Memory Leaks - UnreliableCode Forums

BytePusher

Memory & Performance

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3y ago · Apr 12, 2023 11:20 AM

#1

Hey guys! I wanted to write a quick guide on using

CODE

ArrayPool<T>.Shared

in high-throughput network and file I/O services.

1. The Core Benefit:
Instead of allocating a new

CODE

new byte[4096]

on every incoming packet and triggering Gen0 GC sweeps, you rent a pre-allocated array from the shared pool:

CSHARP

byte[] buffer = ArrayPool<byte>.Shared.Rent(minimumLength: 4096);
try
{
    int bytesRead = await stream.ReadAsync(buffer.AsMemory(0, 4096), cancellationToken);
    ProcessPacket(buffer.AsSpan(0, bytesRead));
}
finally
{
    // Always return in a finally block!
    ArrayPool<byte>.Shared.Return(buffer, clearArray: false);
}

2. Two Big Gotchas to Watch Out For:
- Rented arrays can be larger than requested:

CODE

Rent(4096)

might return an array of length 8192! Always track your actual

CODE

bytesRead

count and use

CODE

buffer.AsSpan(0, count)

, never

CODE

buffer.Length

.
- clearArray Parameter: Set

CODE

clearArray: true

if the buffer contained sensitive user passwords or tokens so subsequent renters cannot read stale memory!

Hey guys! I wanted to write a quick guide on using [code]ArrayPool<T>.Shared[/code] in high-throughput network and file I/O services.

[b]1. The Core Benefit:[/b]
Instead of allocating a new [code]new byte[4096][/code] on every incoming packet and triggering Gen0 GC sweeps, you rent a pre-allocated array from the shared pool:

[code=csharp]byte[] buffer = ArrayPool<byte>.Shared.Rent(minimumLength: 4096);
try
{
    int bytesRead = await stream.ReadAsync(buffer.AsMemory(0, 4096), cancellationToken);
    ProcessPacket(buffer.AsSpan(0, bytesRead));
}
finally
{
    // Always return in a finally block!
    ArrayPool<byte>.Shared.Return(buffer, clearArray: false);
}[/code]

[b]2. Two Big Gotchas to Watch Out For:[/b]
- [b]Rented arrays can be larger than requested:[/b] [code]Rent(4096)[/code] might return an array of length 8192! Always track your actual [code]bytesRead[/code] count and use [code]buffer.AsSpan(0, count)[/code], never [code]buffer.Length[/code].
- [b]clearArray Parameter:[/b] Set [code]clearArray: true[/code] if the buffer contained sensitive user passwords or tokens so subsequent renters cannot read stale memory!

BytePusher · Memory & Performance
Zero-allocation C# code using Span<T>, Memory<T>, and Unsafe...

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CSharpNinja

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3y ago · Apr 12, 2023 2:28 PM

#2

Great writeup @BytePusher! Another subtle bug I see developers make: returning the buffer while another background thread or async task is still holding a reference to it.

If you return a rented buffer early, another thread might rent the exact same buffer and overwrite your data mid-flight. Using

CODE

IMemoryOwner<T>

with

CODE

MemoryPool<T>.Shared

helps enforce ownership lifetimes with standard

CODE

using

blocks:

CSHARP

using IMemoryOwner<byte> owner = MemoryPool<byte>.Shared.Rent(4096);
Memory<byte> memory = owner.Memory;

CSharpNinja · Senior .NET Developer
C# enthusiast, building distributed backend services and hig...

DevDan

.NET Core & Cloud

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3y ago · Apr 12, 2023 4:28 PM

#3

Also worth noting:

CODE

ArrayPool<T>.Shared

uses power-of-two bucket tiers (e.g. 1KB, 2KB, 4KB, 8KB up to 1MB). If you request anything over 1MB, it allocates directly on the Large Object Heap (LOH) without pooling.

For 99% of web API and game server buffers under 64KB, ArrayPool reduced our memory allocations by over 90%!

DevDan · .NET Core & Cloud
Writing clean C# code and microservices since .NET Core 2.1....

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Renting Buffers with ArrayPool<T>.Shared: Common Gotchas, Sizing & Memory Leaks

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