Memory optimisation for GetLastActiveSpan

[jamescrosswell]

#skip-changelog

Overview

When load testing an ASP.NET Core application that creates a lot of spans, GetLastActiveSpan was responsible for a significant proportion of memory allocation:

                    // The endpoint being load tested
                    endpoints.MapGet("/spans", async context =>
                    {
                        for (int i = 0; i < 1000; i++)
                        {
                            var span = SentrySdk.GetSpan()?.StartChild($"child {i}");
                            span?.Finish();
                        }
                        await context.Response.WriteAsync("Hello Spans!");
                    });

This appears to be due to the call to Spans.OrderByDescending:

sentry-dotnet/src/Sentry/TransactionTracer.cs

Line 381 in bbe4fa9

Spans.OrderByDescending(x => x.StartTimestamp).FirstOrDefault(s => !s.IsFinished);

Iteration 1

By replacing this with a for loop, memory consumption in our benchmark dropped by 65.68% from 2109.85 KB to 724.02 KB

Before

Method	SpanCount	Mean	Error	StdDev	Gen0	Gen1	Allocated
'Create spans for scope access'	1	45.39 us	13.71 us	0.752 us	5.6152	1.7700	19.77 KB
'Create spans for scope access'	10	205.46 us	276.93 us	15.179 us	23.9258	4.3945	85.51 KB
'Create spans for scope access'	100	4,055.16 us	5,006.05 us	274.399 us	601.5625	85.9375	2109.85 KB

After

Method	SpanCount	Mean	Error	StdDev	Gen0	Gen1	Allocated
'Create spans for scope access'	1	52.05 us	242.80 us	13.309 us	5.2490	1.6479	18.92 KB
'Create spans for scope access'	10	164.61 us	55.00 us	3.015 us	12.2070	4.1504	63.88 KB
'Create spans for scope access'	100	2,287.86 us	1,098.36 us	60.205 us	183.5938	50.7813	724.02 KB

Iteration 2

By instead keeping all the spans on a Stack and inspecting the most recent one(s) to return the last active span when required we were able to reduce memory usage for the 100 spans scenario drops by a further 63.25% from 724.02 KB to 266.09 KB. So about an ~87% reduction vs the original benchmarks.

After

Method	SpanCount	Mean	Error	StdDev	Gen0	Gen1	Allocated
'Create spans for scope access'	1	28.45 us	13.270 us	0.727 us	4.6387	1.5869	16.64 KB
'Create spans for scope access'	10	84.98 us	6.361 us	0.349 us	9.3994	2.5635	39.17 KB
'Create spans for scope access'	100	657.54 us	54.581 us	2.992 us	57.6172	16.6016	266.09 KB

[jamescrosswell]

Important Note

I merged main into this branch and annoyingly it seems to contain a bunch of the changes from #2637... I'm not sure why but it makes this PR really hard to review.

As such, I created a new branch and PR for these changes... will close this PR in favour of that branch.

See #2642 instead.