Use GC.AllocateUninitializedArray in File.ReadAllBytes[Async] by Copilot · Pull Request #132398 · dotnet/runtime

Copilot · 2026-08-17T10:45:33Z

File.ReadAllBytes and File.ReadAllBytesAsync allocate their result buffer with new byte[count], paying for zeroing that the read loop immediately overwrites. Idea by @GrabYourPitchforks.

Changes

File.ReadAllBytes and File.InternalReadAllBytesAsync now allocate the result via GC.AllocateUninitializedArray<byte>(count).

int count = (int)fileLength;
byte[] bytes = GC.AllocateUninitializedArray<byte>(count);

Safety: both loops fill the entire buffer and throw EndOfFileException if the file yields fewer bytes than its reported length, so the array is either fully written or discarded with the exception — callers can never observe uninitialized memory. GC.AllocateUninitializedArray falls back to a zeroed allocation below its internal size threshold, so only large reads are affected.

Note: the issue refers to GC.GetUninitializedArray; the actual API is GC.AllocateUninitializedArray<T>.

Benchmarks

BenchmarkDotNet, comparing both allocation strategies in otherwise identical RandomAccess-based read loops (in-process toolchain, ShortRun, Linux x64, .NET 11 preview, noisy CI-class machine):

Method	FileSize	Mean	Ratio
Zeroed	1024	2.735 us	1.00
Uninitialized	1024	2.819 us	1.03
ZeroedAsync	1024	22.016 us	1.00
UninitializedAsync	1024	21.328 us	0.97
Zeroed	100000	41.690 us	1.00
Uninitialized	100000	46.091 us	1.11
ZeroedAsync	100000	91.507 us	1.00
UninitializedAsync	100000	93.875 us	1.03
Zeroed	10000000	1,579.992 us	1.00
Uninitialized	10000000	1,359.842 us	0.86
ZeroedAsync	10000000	1,816.723 us	1.00
UninitializedAsync	10000000	1,388.709 us	0.76

10 MB reads are ~14% (sync) / ~24% (async) faster; smaller sizes fall within noise.

Note

This pull request was created by GitHub Copilot.

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

azure-pipelines · 2026-08-17T10:47:03Z

Azure Pipelines: Successfully started running 3 pipeline(s). 13 pipeline(s) were filtered out due to trigger conditions. There may be pipelines that require an authorized user to comment /azp run to run.

dotnet-policy-service · 2026-08-17T10:49:55Z

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

Copilot

Pull request overview

This pull request optimizes System.IO.File.ReadAllBytes and File.ReadAllBytesAsync in System.Private.CoreLib by avoiding the cost of zero-initializing the destination buffer when the subsequent read loop overwrites the entire array.

Changes:

Switch ReadAllBytes to allocate its result buffer via GC.AllocateUninitializedArray<byte>(count) instead of new byte[count].
Switch the async implementation path (InternalReadAllBytesAsync) to allocate via GC.AllocateUninitializedArray<byte>(count) as well.
Add inline comments documenting why skipping zero-init is safe due to full-buffer overwrite and EOF exception behavior.

adamsitnik · 2026-08-17T12:12:34Z

@EgorBot -osx_arm64 -linux_x64 -windows_x64

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Benchmarks).Assembly).Run(args);

[MemoryDiagnoser]
public class Benchmarks
{
    private string? _filePath;

    [Params(1024, 100_000, 10_000_000)]
    public int FileSize;

    [GlobalSetup]
    public void Setup()
    {
        _filePath = Path.GetTempFileName();
        byte[] content = new byte[FileSize];
        new Random(42).NextBytes(content);
        File.WriteAllBytes(_filePath, content);
    }

    [GlobalCleanup]
    public void Cleanup() => File.Delete(_filePath!);

    [Benchmark]
    public byte[] Sync() => File.ReadAllBytes(_filePath!);

    [Benchmark]
    public Task<byte[]> Async() => File.ReadAllBytesAsync(_filePath!);
}

azure-pipelines · 2026-08-17T12:13:26Z

Azure Pipelines: Successfully started running 3 pipeline(s). 13 pipeline(s) were filtered out due to trigger conditions. There may be pipelines that require an authorized user to comment /azp run to run.

adamsitnik

LGTM. The benchmark results show a 3-6% performance improvement on Unix (macOS and Linux) for medium and large files. For small files, and on Windows in general, the difference is within the margin of error.

Details

Results for `macos26_helix_arm64`

@adamsitnik

BenchmarkDotNet v0.16.0-preview.1, macOS Sequoia 15.4.1 (24E263) [Darwin 24.4.0]
Apple M4, 1 CPU, 10 logical and 10 physical cores
Memory: 16 GB Total, 4.54 GB Available
.NET SDK 11.0.100-rc.1.26413.103
  [Host]     : .NET 11.0.0 (11.0.0-rc.1.26413.103, 11.0.26.41403), Arm64 RyuJIT armv8.0-a

Method	Toolchain	FileSize	Mean	Error	Ratio	Gen0	Gen1	Gen2	Allocated	Alloc Ratio
Sync	main**	1024	7.471 μs	0.0381 μs	1.01	0.1297	-	-	1.08 KB	1.00
Sync	PR #132398	1024	7.408 μs	0.0276 μs	1.00	0.1297	-	-	1.08 KB	1.00

Async	main	1024	9.767 μs	0.0385 μs	1.00	0.1678	-	-	1.38 KB	1.00
Async	PR #132398	1024	9.731 μs	0.0509 μs	1.00	0.1678	-	-	1.38 KB	1.00

Sync	main**	100000	17.348 μs	0.0929 μs	1.01	31.2195	31.2195	31.2195	97.76 KB	1.00
Sync	PR #132398	100000	17.158 μs	0.1232 μs	1.00	31.2195	31.2195	31.2195	97.76 KB	1.00

Async	main	100000	16.556 μs	0.0899 μs	1.04	31.2195	31.2195	31.2195	98.06 KB	1.00
Async	PR #132398	100000	15.899 μs	0.1585 μs	1.00	31.2195	31.2195	31.2195	98.06 KB	1.00

Sync	main**	10000000	733.256 μs	6.7370 μs	1.03	166.9922	166.9922	166.9922	9767.1 KB	1.00
Sync	PR #132398	10000000	713.734 μs	6.2004 μs	1.00	224.6094	224.6094	224.6094	9767.61 KB	1.00

Async	main	10000000	667.484 μs	3.6360 μs	1.16	150.3906	150.3906	150.3906	9767.01 KB	1.00
Async	PR #132398	10000000	575.771 μs	7.2609 μs	1.00	109.3750	109.3750	109.3750	9766.69 KB	1.00

Full logs

Results for `ubuntu24_azure_turin`

@adamsitnik

BenchmarkDotNet v0.16.0-preview.1, Linux Ubuntu 24.04.4 LTS (Noble Numbat)
AMD EPYC 9V45 2.60GHz, 1 CPU, 8 logical and 4 physical cores
Memory: 31.34 GB Total, 10.28 GB Available
.NET SDK 11.0.100-rc.1.26413.103
  [Host]     : .NET 11.0.0 (11.0.0-rc.1.26413.103, 11.0.26.41403), X64 RyuJIT x86-64-v4

Method	Toolchain	FileSize	Mean	Error	Ratio	Gen0	Gen1	Gen2	Allocated	Alloc Ratio
Sync	main**	1024	5.531 μs	0.0499 μs	1.01	0.0610	-	-	1.08 KB	1.00
Sync	PR #132398	1024	5.458 μs	0.0456 μs	1.00	0.0610	-	-	1.08 KB	1.00

Async	main	1024	7.303 μs	0.1409 μs	0.99	0.0763	-	-	1.37 KB	1.00
Async	PR #132398	1024	7.349 μs	0.1433 μs	1.00	0.0839	-	-	1.38 KB	1.00

Sync	main**	100000	44.881 μs	0.3425 μs	0.98	31.1890	31.1890	31.1890	97.76 KB	1.00
Sync	PR #132398	100000	45.630 μs	0.5886 μs	1.00	31.1890	31.1890	31.1890	97.76 KB	1.00

Async	main	100000	12.532 μs	0.2492 μs	1.06	31.2347	31.2347	31.2347	98.06 KB	1.00
Async	PR #132398	100000	11.877 μs	0.2354 μs	1.00	31.2347	31.2347	31.2347	98.06 KB	1.00

Sync	main**	10000000	608.650 μs	5.2426 μs	1.06	500.0000	500.0000	500.0000	9766.11 KB	1.00
Sync	PR #132398	10000000	571.953 μs	6.8419 μs	1.00	500.0000	500.0000	500.0000	9766.11 KB	1.00

Async	main	10000000	1,144.992 μs	4.6155 μs	1.06	396.4844	396.4844	396.4844	9767.52 KB	1.00
Async	PR #132398	10000000	1,078.073 μs	21.3568 μs	1.00	396.4844	396.4844	396.4844	9769.31 KB	1.00

Full logs

Results for `windows_azure_turin`

@adamsitnik


BenchmarkDotNet v0.16.0-preview.1, Windows 11 (10.0.26100.33296/24H2/2024Update/HudsonValley) (Hyper-V)
AMD EPYC 9V45 2.60GHz, 1 CPU, 8 logical and 4 physical cores
Memory: 31.99 GB Total, 27.01 GB Available
.NET SDK 11.0.100-rc.1.26413.103
  [Host]     : .NET 11.0.0 (11.0.0-rc.1.26413.103, 11.0.26.41403), X64 RyuJIT x86-64-v4

Method	Toolchain	FileSize	Mean	Error	Ratio	Gen0	Gen1	Gen2	Allocated	Alloc Ratio
Sync	main**	1024	22.55 μs	0.364 μs	1.02	0.0610	-	-	1.1 KB	1.00
Sync	PR #132398	1024	22.12 μs	0.315 μs	1.00	0.0610	-	-	1.1 KB	1.00

Async	main	1024	35.10 μs	0.669 μs	1.01	0.0610	-	-	1.61 KB	1.00
Async	PR #132398	1024	34.66 μs	0.456 μs	1.00	0.0610	-	-	1.61 KB	1.00

Sync	main**	100000	57.16 μs	0.689 μs	1.12	31.1890	31.1890	31.1890	97.78 KB	1.00
Sync	PR #132398	100000	51.09 μs	0.581 μs	1.00	31.1890	31.1890	31.1890	97.78 KB	1.00

Async	main	100000	39.72 μs	0.400 μs	1.02	31.1890	31.1890	31.1890	98.29 KB	1.00
Async	PR #132398	100000	38.92 μs	0.759 μs	1.00	31.1890	31.1890	31.1890	98.29 KB	1.00

Sync	main**	10000000	1,701.06 μs	33.238 μs	1.00	500.0000	500.0000	500.0000	9766.13 KB	1.00
Sync	PR #132398	10000000	1,699.38 μs	33.987 μs	1.00	500.0000	500.0000	500.0000	9766.13 KB	1.00

Async	main	10000000	2,419.92 μs	79.170 μs	1.01	390.6250	390.6250	390.6250	9768 KB	1.00
Async	PR #132398	10000000	2,395.01 μs	47.691 μs	1.00	390.6250	390.6250	390.6250	9769.52 KB	1.00

Full logs

adamsitnik · 2026-08-17T16:19:35Z

/ba-g networking failures, unrelated

Use GC.AllocateUninitializedArray in File.ReadAllBytes[Async]

5186426

Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI self-assigned this Aug 17, 2026

Copilot AI lite review requested due to automatic review settings August 17, 2026 10:45

Copilot AI assigned adamsitnik Aug 17, 2026

Copilot started reviewing on behalf of adamsitnik August 17, 2026 10:46 View session

Copilot finished work on behalf of adamsitnik August 17, 2026 10:46

Copilot AI requested a review from adamsitnik August 17, 2026 10:46

Copilot created this pull request from a session on behalf of adamsitnik August 17, 2026 10:46 View session

github-actions Bot added the area-System.IO label Aug 17, 2026

Copilot AI reviewed Aug 17, 2026

View reviewed changes

jkotas added the tenet-performance Performance related issue label Aug 17, 2026

adamsitnik marked this pull request as ready for review August 17, 2026 12:12

EgorBot mentioned this pull request Aug 17, 2026

Benchmarks for dotnet/runtime#132398 (for @adamsitnik) EgorBot/Benchmarks#500

Open

EgorBo reviewed Aug 17, 2026

View reviewed changes

Comment thread src/libraries/System.Private.CoreLib/src/System/IO/File.cs

adamsitnik approved these changes Aug 17, 2026

View reviewed changes

adamsitnik requested review from GrabYourPitchforks and jkotas August 17, 2026 13:28

jkotas approved these changes Aug 17, 2026

View reviewed changes

build-analysis Bot mentioned this pull request Aug 17, 2026

System.Net.Tests.InvalidClientRequestTests.GetContext_InvalidRequest_DoesNotGetContext times out #132336

Open

adamsitnik merged commit 185098c into main Aug 17, 2026
134 of 139 checks passed

adamsitnik deleted the copilot/update-file-readallbytes-async branch August 17, 2026 16:21

adamsitnik added this to the 11.0.0 milestone Aug 17, 2026

Navigation Menu

Uh oh!

Use GC.AllocateUninitializedArray in File.ReadAllBytes[Async] - #132398

Use GC.AllocateUninitializedArray in File.ReadAllBytes[Async]#132398
adamsitnik merged 1 commit into
mainfrom
copilot/update-file-readallbytes-async

Copilot AI commented Aug 17, 2026 •

edited

Loading

Uh oh!

azure-pipelines Bot commented Aug 17, 2026

Uh oh!

dotnet-policy-service Bot commented Aug 17, 2026

Uh oh!

Copilot AI left a comment

Uh oh!

adamsitnik commented Aug 17, 2026

Uh oh!

azure-pipelines Bot commented Aug 17, 2026

Uh oh!

Uh oh!

adamsitnik left a comment •

edited

Loading

Uh oh!

adamsitnik commented Aug 17, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

Uh oh!

Conversation

Copilot AI commented Aug 17, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Changes

Benchmarks

Uh oh!

azure-pipelines Bot commented Aug 17, 2026

Uh oh!

dotnet-policy-service Bot commented Aug 17, 2026

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Uh oh!

adamsitnik commented Aug 17, 2026

Uh oh!

azure-pipelines Bot commented Aug 17, 2026

Uh oh!

Uh oh!

adamsitnik left a comment • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Results for macos26_helix_arm64

Results for ubuntu24_azure_turin

Results for windows_azure_turin

Uh oh!

adamsitnik commented Aug 17, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

Copilot AI commented Aug 17, 2026 •

edited

Loading

adamsitnik left a comment •

edited

Loading

Results for `macos26_helix_arm64`

Results for `ubuntu24_azure_turin`

Results for `windows_azure_turin`