Trying to use Span to save money...and falling short

Written on April 12, 2019

Learn through doing… failing and trying again

For me the path to enlightenment is usually through diving in and trying to build something. I was skeptical about posting this as ultimately it turned out to be a fail, but if this post helps just helps one person then I’ll call it a win.

Spin Span to win!

I had been curious to try out the new Span features of .NET Core since they were released with 2.1, if you need a refresh then this post should help.

I had done some reading and kept hearing about how these features were improving performance of the underlying BCL. Then I read about how .NET Core 3.0 would be replacing Newtonsoft with a new set of JSON APIs and part of the rational was around being able to use Span for high performance throughput, detailed info on that can be found in this [GitHub issue].

Great, I could use some high performance and lower memory usage to shave some $’s of my serverless bill. The major cloud providers charge based on GB/s for compute time, so the less of these I use the cheaper the bill.

I even had a function to refactor, we had a notification that needed to be persisted for auditing purposes. The notification needed to be parsed to extract some relevant data for later reporting. The current function was using Substring to extract a file name from a path. I could replace this with Span and Slice and save on some allocations. Steve Gordon actually did a great post about this with an example of how to get allocations down to 0 and benchmarks this using BenchmarkDotNet!

.ToString or not .ToString

So I built a little demo to reproduce the sample environment and benchmark. All was going well and the initial cut I was able to get to zero allocations, but then I tried to create the output as an object!

String is immutable! That is all!

    public Event CreateEventWithSpan(EventGridEvent eventGridEvent)
    {
        ReadOnlySpan<char> subject = eventGridEvent.Subject;
        var lastSpaceIndex = subject.LastIndexOf('/');
        return new Event()
        {
            Id = Guid.NewGuid().ToString(),
            BlobName = subject.Slice(lastSpaceIndex + 1).ToString(),
            EventData = eventGridEvent.Data
        };
    }

So in order to get my parsed data back out into an object that I could persist in the database I have to call .ToString() on the Span… which allocates memory… which defeats the point and disproves my hypothesis!

So I did a fair bit of digging on this and really came to the conclusion that I was doing it wrong, a better use case would be operating over a stream. But it is great to see more of the underlying APIs using this and I can see this will have a big improvement for the JSON API’s in .NET 3.0.

I learnt heaps doing this and that is what I am taking away.

Written on April 12, 2019