Memory Management, C# 7.2 and Span<T>

HELLO! F# |> I ❤ 9 years of experience in .NET Organiser of Bristol F# meetup and DDD South West You can find me at @stuartblang & https://stu.dev

Rust

● FAST ● SAFE ● MAINTAINABLE Can C# have all 3?

C# as a high performance language

Performance where it matters Fr am ew or ks Li br ar ie s

Always Measure

Measure with… ▸ ▸ ▸ ▸ Memory Performance Counters BenchmarkDotNet dotMemory Visual Studio

How it works

“ The Stack Is An Implementation Detail. - Eric Lippert Part One Part Two

Types ▸ Value types ▹ Structs ▹ Enums ▹ Bool ▹ Int, Float, Short, Long… ▹ Byte ▸ Reference types ▹ Class ▹ Interface ▹ Array ▹ String ▹ Delegate

Stack Code sharplab.io demo a=42 Stack Heap x(ref) object: array{1,2,3}

Code - behaviour

Stack Credit: http://www.i-programmer.info/ebooks/deep-c/363

Heap Small Object Heap Gen 0 Gen 1 Large Object Heap Gen 2

Heap Credit: https://www.dynatrace.com/resources/ebooks/javabook/how-garbage-collection-works/

Great Resource ▸ http://benhall.io/a-super-simplified-exp lanation-of-net-garbage-collection/

Stack vs Heap What? Lifetime When disposed? Time to dispose Stack Local values Local references (just the ref part) Stack frame Deterministic Constant (near instant) Heap Reference object instances Unrestricted GC non-deterministic It depends (non-trivial)

Watch out for hidden allocations ▸ Let’s see some examples

Watch out for hidden allocations

Plugins ▸ R# Heap Allocations Viewer ▸ Roslyn Clr Heap Allocation Analyzer

In Parameters

In Parameters - cont. ▸ Essentially readonly ref ▸ Where you want to want to pass by-ref for performance, with the safety and behaviour or by-value ▸ Watch out for copying with invoking methods on non-readonly structs

Ref extension methods

Ref locals ▸ Reference semantics with value types

Ref locals ▸ Can be thought of as aliases

Ref returns

Ref Struct ▸ Aka ref-like ▸ Stack-only! ▸ To support Span<T>

Ref Struct

“ The great Eric Lippert once wrote “The Stack Is An Implementation Detail”, and basically that’s not true anymore. - Jon Skeet

Span<T> ▸ Abstraction over arbitrary memory Span<T> Marshal.AllocHGlobal() stackalloc [] new []

Span<T>

Span<T> - Why stack-only? ▸ Safe lifetime - cannot outlive stack memory ▸ Safe concurrency - No struct tearing ▸ It’s fast!

Span<T> - Other properties ▸ Efficient representation ▸ GC tracking - refs will be maintained by GC

Use Cases ▸ Alternative to unsafe code ▸ Parsing text ▸ Sharing partial parts of memory

Problem with pointers ▸ ▸ ▸ ▸ Requires GC pinning (for heap memory) Mustn’t escape pointer (for stack) No bounds checking Unsafe code blocks

Problem with pointers

Credit: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/stackalloc

You can still pin

You can still pin - in C# 7.3

BenchmarkDotNet

Results

C# 8 - Slicing with Ranges demo

Span<T> Framework APIs Today netcoreapp2.0 ❌ netcoreapp2.1 ✅ netstandard2.0 ❌ Next netcoreapp2.2 ✅ netcoreapp2.3 ✅ netcoreapp3.0 ✅ netstandard2.1 ✅ ❓ net472 ❌ net48 ❌ net49 ❓

Related topics ▸ ReadOnlySpan<T> ▸ Memory<T> & ReadOnlyMemory<T> ▸ System.Runtime.CompilerServices.Unsafe ▸ MemoryMarshal.AsBytes(span) ▸ MemoryMarshal.Cast<TFrom,TTo>(span)

Libraries using Span<T> ( just some) ▸ Utf8Json ▸ SpanJson ▸ ZeroFormatter ▸ StackExchange.Redis

Case study JustEat.StatsD PRs Low hanging fruit: #55, #59, #63, #65 Master class by dv00d00: #104 - Zero Allocations

Good Reads Adam Sitnik - Span Marc Gravell - Spans and ref Parts 1 & 2 Span<T> spec Vladimir Sadov All About Span: Exploring a New .NET Mainstay Stephen Toub ▸ Maarten Balliauw ▸ ▸ ▸ ▸ ▸

Summary

THANKS! Any questions? You can find me at @stuartblang