Safe Zero-Copy Operations in C#

Somebody decided that "safety" included rejecting obvious concepts like this.

In my experience, the biggest wins by far were achieved by using the network tab of the browser F12 tools. The next biggest was Azure Application Insights profiler running in production. Look at the top ten most expensive database queries and tune them to death.

The use of Span<T> and the like is much more important for the authors of shared libraries more than "end users" writing a web app. Speaking of which, you can increase your usage of it by simply updating your NuGet package versions, .NET framework version to 9 or 10, etc... This will provide thousands of such micro optimisations for very little effort!

You really need to measure before going to low level optimizations like this. Odds are in this case that the overhead is in the framework/CMS, and you gain the most by understanding how it works and how to use it better.

Span<T> is really more of an optimization you should pay attention to when you write lower level library code.

The two .NET CMS I have experience with, Sitecore and Optimizely, something like Span would hardly bring any improvement, rather check the way their ORM is being used, do some direct SQL, cache some renderings in a different way, cross check if the CMS APIs are being correctly used.

Most of the benefits of Span<T> you gain by keeping up with .NET upgrades. Span<T> is a low level optimization that benefits things like ASP.NET internals far more than most user code. Each version of .NET since Span<T> was added has improved the use of it. Additionally in C#, the compiler prefers Span<T> overloads when they make sense so just rebuilding for the most recent .NET opts you in to the benefits. Whether or not those are "serious" benefits is a matter of taste and also a reminder that your code probably doesn't spend all of its time doing low level things. (Your database query time, for instance is generally going to have a bigger impact.)

I'd add a big word of caution for "public readonly record struct". I've seen a few codebases start using that wordy version "by default" and then build themselves into a far bigger terrible performance pit than they expected. There's a lot of good reasons that "record" defaults to class and has you opt in to struct behavior. It's a lot easier to reason about classes. It's a lot easier to understand the performance trade-offs on the side of classes. The GC is your friend, not your enemy, even and sometimes especially for short-lived data. (Gen0 collections are often very fast. The "nursery" was designed for fire-and-forget data churn. It's the bread-and-butter job of a generational garbage collector to separate the churn from the stable, handle the churn quickly and keep the stable stabler.)

Structs are pass-by-value, which means as soon as they exit the "lucky path" of staying in the same stack they are copied from place to place. If your models include a lot of other structs, you start copying memory a lot more regularly. If your structs grow too large for certain stackframe quotas they get boxed onto the GC heap anyway not saving you heap allocations.

Classes are pass-by-reference. If you are using "readonly" as a part of your structs to build immutable data models, all the copies add up from every immutable data change creating a new struct. Whereas "regular' immutable records (classes) can share structure between each other by reference (the immutable parts that don't change can use the same references and thus share the same memory).

If your models are more than a couple integers and have any sort of nesting or complex relationships, "public readonly record struct" can be a premature optimization that actually ends up costing you performance. Not every bit of data can be moved to the stack and not every bit of data should be moved to the stack. Keep in mind there are trade-offs and a healthy performing .NET application generally uses a smart mixture of stack and GC, because they are both important tools in the toolbelt. Like I said, there are reasons that "public record" defaults to "class" and "public readonly record struct" is the wordy opt-in and it is useful to keep them in mind.

This response is not directly answering that "in a .NET CMS" part of your question. I'm just trying to say how to think about when to worry about optimizations.

These sorts of micro optimizations are best considered when your are trying to solve a particular performance problem, particularly when you are dealing with a site that is not getting a lot of hits. I've experienced using small business ecommerce websites where each page load takes 5 seconds and given up trying to buy something. In that case profiling the site and figuring out the problem is very worth while.

When you have a site getting a lot of hits, these sorts of performance optimizations can help you save cost. If your service takes 100 servers to run and you can find some performance tweaks to get down to 75 server, that may be worth the engineering effort.

My recommendation is to use a profiler of some type. Either on your application in aggregate to identify hot spots in in search of the source of a particular performance problem. Once you identify a hot spot, construct a micro benchmark of the problem in BenchmarkDotNet and try to use tools like Span<T> to fix the problem.

Span<T> , stackalloc and value-structs will matter more when writing heavy data/number crunching scenarios like imageprocessing, games, "AI"/vector queries or things like _implementing_ database engines (see yesterdays discussion on the guys announcing they're using C++ where Rust, Go, Erlang, Java and C# was discussed for comparisons https://news.ycombinator.com/item?id=45389744 ).

I'm often spending my days on writing applications that are reminiscent of CMS workloads and while I sometimes do structs, I've not really bought out my lowlevel optimization skills more than a few times in the past 6 years, 95% of the time it's bad usage of DB's.

For example, Rust allows the equivalent of storing `Span<T>` (called slice in Rust) everywhere (including on the heap, although this is rare).

C# is GC'd, the system will protect memory in use and while it can allow things like Span<>, Memory<>,etc there are some constraints due to sloppy lifetime reasoning but in general "easy" usage since you do not need to care about lifetime.

Rust has lifetime semantics built down to the core of the language, the compiler will know much better about what's safe but also forbid you early from things that are safe but not provable (they're improving the checkers based on experience though), due to it's knowledge it will be better at handling allocations more exactly.

Personally as someone with an assembly, C,C++,etc background, while I see the allure of Rust and do see it as a plus if less experienced devs that really need perf go for Rust for critical components, and thinking I'm going to try to do some project in Rust...

I've so far not seen a project where the slight performance improvement of Rust will outweight the "productivity" gain from being able to do a bit "sloppy" connections that C# allows for.

“A comparison of Rust’s borrow checker to the one in C#”

https://em-tg.github.io/csborrow/

C# _can_ do this! But I face many abstractions: special perf APIs, C#, IL, asm. Outcomes will vary with language version, runtime version, platform, IL2CPP/Burst/Mono/dotnet. But C/C++ has one layer of abstraction (the compiler), and it's locked in once I compile it.

I want to do the thing as exactly and consistently as possible in the simplest way possible!

A build environment that compiles .cpp alongside .cs (no automatic bindings, just compilation) would be so nice for this.

----

Example of what I mean regarding abstractions:

  void addBatch(int *a, int *b, int count)
  {
    for(int i=0; i<count; i++) 
      a[i] += b[i]; 
  }

    [MethodImpl(MethodImplOptions.AggressiveOptimization)]
    public static void AddBatch(int[] a, int[] b, int count)
    {
        ref int ra = ref MemoryMarshal.GetArrayDataReference(a);
        ref int rb = ref MemoryMarshal.GetArrayDataReference(b);
        for (nint i = 0, n = (nint)count; i < n; i++)
            Unsafe.Add(ref ra, i) += Unsafe.Add(ref rb, i);
    }

Then people go around shouting GC's are bad because they used them with a language that made them use some of the worst ones out there.

To work with strings you should use StringBuilder.