Regression in File.PreallocationSize

[performanceautofiler]

Run Information

Architecture	x64
OS	ubuntu 18.04
Baseline	a8c2d1eae1726b1e6ec50716b7d4c34bc21ba96e
Compare	ea062deec80053b822734802b40642bea36bed33
Diff	Diff

Regressions in System.IO.Tests.Perf_FileStream

Benchmark	Baseline	Test	Test/Base	Test Quality	Edge Detector	Baseline IR	Compare IR	IR Ratio	Baseline ETL	Compare ETL
Write_NoBuffering_PreallocationSize - Duration of single invocation	34.65 ms	41.07 ms	1.19	0.05	True

Test Report

Repro

git clone https://github.com/dotnet/performance.git
python3 .\performance\scripts\benchmarks_ci.py -f net6.0 --filter 'System.IO.Tests.Perf_FileStream*'

Payloads

Baseline
Compare

Histogram

System.IO.Tests.Perf_FileStream.Write_NoBuffering_PreallocationSize(fileSize: 104857600, userBufferSize: 16384, options: None)

Docs

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

### Run Information

Architecture	x64
OS	ubuntu 18.04
Baseline	a8c2d1eae1726b1e6ec50716b7d4c34bc21ba96e
Compare	ea062deec80053b822734802b40642bea36bed33
Diff	Diff

Regressions in System.Text.Json.Tests.Perf_Booleans

Benchmark	Baseline	Test	Test/Base	Test Quality	Edge Detector	Baseline IR	Compare IR	IR Ratio	Baseline ETL	Compare ETL
WriteBooleans - Duration of single invocation	1.23 ms	1.46 ms	1.18	0.01	False

Test Report

Repro

git clone https://github.com/dotnet/performance.git
python3 .\performance\scripts\benchmarks_ci.py -f net6.0 --filter 'System.Text.Json.Tests.Perf_Booleans*'

Payloads

Baseline
Compare

Histogram

System.Text.Json.Tests.Perf_Booleans.WriteBooleans(Formatted: False, SkipValidation: True)

Docs

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

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[kunalspathak]

Introduced by #59338

[kunalspathak]

The regression in System.Text.Json.Tests.Perf_Booleans also points to the #59338 , so please double check if that was the cause.

[kunalspathak]

alpine regression - dotnet/perf-autofiling-issues#1590

[stephentoub]

cc: @tmds

[tmds]

Introduced by #59338

The PR made a functional change in preallocationSize and that impacts Write_NoBuffering_PreallocationSize benchmark.
The functional change has the desired behavior.

The regression in System.Text.Json.Tests.Perf_Booleans also points to the #59338 , so please double check if that was the cause.

Perf_Booleans doesn't seem to use preallocationSize, so I don't think it has the same root cause.

[stephentoub]

The PR made a functional change in preallocationSize and that impacts Write_NoBuffering_PreallocationSize benchmark.

Do we know why it had such an impact? Is the syscall being used now 20% slower?

[tmds]

Do we know why it had such an impact? Is the syscall being used now 20% slower?

The posix_fallocate and fallocate function both use the fallocate syscall. The difference is that we're now calling it with the FALLOC_FL_KEEP_SIZE to preserve the length. In theory, this means the kernel has less work since it doesn't need to care about 'filling the space', but that is not what the benchmark is telling us.

I don't think there is anything we can do about improving the runtime implementation.

For the benchmark, it uses tmpfs, though for preallocation it would be interesting to benchmark on an actual disk. I'm not sure how stable the results would be, but maybe the benchmarking framework accounts for some variance and outliers.

I'll run the benchmark locally and see if I learn something from that. I may not get to it immediately.