[Perf -11%] System.Buffers.Tests.ReadOnlySequenceTests<Char>.IterateGetPositionTenSegments

[DrewScoggins]

Run Information

Architecture	x64
OS	ubuntu 18.04
Baseline	2f2593177dafbe702407fe0b7ac156a7829b7ee6
Compare	6cf1b8ec012d52880d46fa4773f60ed52ddc9f3d
Diff	Link

Regressions in System.Buffers.Tests.ReadOnlySequenceTests<Char>

Benchmark	Baseline	Test	Test/Base	Baseline IR	Compare IR	IR Ratio	Baseline ETL	Compare ETL
IterateGetPositionTenSegments	63.58 ns	70.75 ns	1.11

Historical Data in Reporting System

Repro

git clone https://github.com/dotnet/performance.git
python3 .\performance\scripts\benchmarks_ci.py -f netcoreapp5.0 --filter 'System.Buffers.Tests.ReadOnlySequenceTests<Char>*'

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Payloads

Baseline
Compare

Histogram

System.Buffers.Tests.ReadOnlySequenceTests.IterateGetPositionTenSegments

[61.840 ; 64.058) | @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
[64.058 ; 65.542) | @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
[65.542 ; 68.087) | @@@@@@@@@
[68.087 ; 69.574) | 
[69.574 ; 71.007) | @@@@@@@@@@@

Docs

Profiling workflow for dotnet/runtime repository
Benchmarking workflow for dotnet/runtime repository

[danmoseley]

No smoking gun but this change is perhaps the most likely relevant in the diff?

f6d8e88

@dotnet/jit-contrib thoughts?

[AndyAyersMS]

Could be. My guess is that that PR altered inlining and from there could be a number of things that impacted perf.

Someone on codegen should follow up.

[danmoseley]

OK, I've moved it to that area.

[SingleAccretion]

I will take a look. There were some positive diffs for SequenceReader on Windows x64...

[SingleAccretion]

I studied this method, taken verbatim (except for the type parameter) from the benchmark, with the unix_x64_x64 AltJit:

[MethodImpl(MethodImplOptions.NoInlining)]
private int IterateGetPosition(ReadOnlySequence<char> sequence)
{
    int consume = 0;

    SequencePosition position = sequence.Start;
    int offset = (int)(sequence.Length / 10);
    SequencePosition end = sequence.GetPosition(0, sequence.End);

    while (!position.Equals(end))
    {
        position = sequence.GetPosition(offset, position);
        consume += position.GetInteger();
    }

    return consume;
}

The (new) folding for it kicks in 12 times during the compilation, but does not affect the inlining, and the final assembly diff, while present, is decidedly non-informative for why the regression is there: https://www.diffchecker.com/aAIwTy3K.

Unfortunately, I do not have a Unix environment on which I could run the real benchmark (and get the actual assembly and perf numbers), so I will not be able to provide much more information on this. It looks like this could be related to alignment, but at the same time, the inner loop is very big.

[danmoseley]

Thank you @SingleAccretion .

I have gotten useful results doing perf measurements on WSL2, if you are interested in that option.

[SingleAccretion]

I will try that option and see how far do I get. It may take a considerable amount of time 😄.