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Follow up on Scatter/Gather API changes #58447

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merged 9 commits into from
Oct 14, 2021
Merged

Follow up on Scatter/Gather API changes #58447

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e502084
Allocate an array of memory handles only if needed.
teo-tsirpanis Aug 31, 2021
3f80275
Remove an unnecessary variable in the multiple-syscall write gather.
teo-tsirpanis Aug 31, 2021
6e0836b
Actually verify the content read by the read scatter operation.
teo-tsirpanis Aug 31, 2021
e95d021
Use AssertExtensions.SequenceEqual.
teo-tsirpanis Aug 31, 2021
8177b23
Delay allocating native memory.
teo-tsirpanis Aug 31, 2021
1450290
Verify that the whole file was read in the scatter/gather test.
teo-tsirpanis Aug 31, 2021
1c74cdf
Test the case when the scatter/gather buffers are acceptable by the W…
teo-tsirpanis Aug 31, 2021
4625c16
Avoid null pointer dereferences when passing an empty segment array.
teo-tsirpanis Sep 13, 2021
0c0f64e
Test performing scatter/gather I/O with an empty segment array.
teo-tsirpanis Sep 13, 2021
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26 changes: 13 additions & 13 deletions src/libraries/System.IO.FileSystem/tests/RandomAccess/NoBuffering.Windows.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -180,18 +180,17 @@ public async Task WriteAsyncUsingMultipleBuffers(bool asyncOperation, bool async
Assert.Equal(content, File.ReadAllBytes(filePath));
}

[Fact]
public async Task ReadWriteAsyncUsingNonPageSizedMultipleBuffers()
[Theory]
[InlineData(true)]
[InlineData(false)]
public async Task ReadWriteAsyncUsingMultipleBuffers(bool memoryPageSized)
{
string filePath = GetTestFilePath();
// The Windows scatter/gather APIs accept segments that are exactly one page long.
// Combined with the FILE_FLAG_NO_BUFFERING's requirements, the segments must also
// be aligned at page size boundaries and have a size of a multiple of the page size.
// Using segments with a length of twice the page size adheres to the second requirement
// but not the first. The RandomAccess implementation will see it and issue sequential
// read/write syscalls per segment, instead of one scatter/gather syscall.
// This test verifies that fallback behavior.
int bufferSize = Environment.SystemPageSize * 2;
// We test with buffers both one and two memory pages long. In the former case,
// the I/O operations will issue one scatter/gather API call, and in the latter
// case they will issue multiple calls; one per buffer. The buffers must still
// be aligned to comply with FILE_FLAG_NO_BUFFERING's requirements.
int bufferSize = Environment.SystemPageSize * (memoryPageSized ? 1 : 2);
int fileSize = bufferSize * 2;
byte[] content = RandomNumberGenerator.GetBytes(fileSize);

Expand All @@ -205,10 +204,11 @@ public async Task ReadWriteAsyncUsingNonPageSizedMultipleBuffers()
await RandomAccess.WriteAsync(handle, new ReadOnlyMemory<byte>[] { firstHalf, secondHalf }, 0);

buffer.GetSpan().Clear();
await RandomAccess.ReadAsync(handle, new Memory<byte>[] { firstHalf, secondHalf }, 0);
}
long nRead = await RandomAccess.ReadAsync(handle, new Memory<byte>[] { firstHalf, secondHalf }, 0);

Assert.Equal(content, await File.ReadAllBytesAsync(filePath));
Assert.Equal(buffer.GetSpan().Length, nRead);
AssertExtensions.SequenceEqual(buffer.GetSpan(), content.AsSpan());
}
}

// when using FileOptions.Asynchronous we are testing Scatter&Gather APIs on Windows (FILE_FLAG_OVERLAPPED requirement)
Expand Down
52 changes: 35 additions & 17 deletions src/libraries/System.Private.CoreLib/src/System/IO/RandomAccess.Windows.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@
using System.Buffers;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.IO.Strategies;
using System.Numerics;
using System.Runtime.InteropServices;
Expand Down Expand Up @@ -437,7 +438,7 @@ private static bool CanUseScatterGatherWindowsAPIs(SafeFileHandle handle)
// The pinned MemoryHandles and the pointer to the segments must be cleaned-up
// with the CleanupScatterGatherBuffers method.
private static unsafe bool TryPrepareScatterGatherBuffers<T, THandler>(IReadOnlyList<T> buffers,
THandler handler, out MemoryHandle[] handlesToDispose, out IntPtr segmentsPtr, out int totalBytes)
THandler handler, [NotNullWhen(true)] out MemoryHandle[]? handlesToDispose, out IntPtr segmentsPtr, out int totalBytes)
where THandler: struct, IMemoryHandler<T>
{
int pageSize = s_cachedPageSize;
Expand All @@ -447,13 +448,11 @@ private static unsafe bool TryPrepareScatterGatherBuffers<T, THandler>(IReadOnly
long alignedAtPageSizeMask = pageSize - 1;

int buffersCount = buffers.Count;
handlesToDispose = new MemoryHandle[buffersCount];
handlesToDispose = null;
segmentsPtr = IntPtr.Zero;
totalBytes = 0;

// "The array must contain enough elements to store nNumberOfBytesToWrite bytes of data, and one element for the terminating NULL. "
long* segments = (long*) NativeMemory.Alloc((nuint)(buffersCount + 1), sizeof(long));
segments[buffersCount] = 0;
long* segments = null;

bool success = false;
try
Expand All @@ -469,18 +468,32 @@ private static unsafe bool TryPrepareScatterGatherBuffers<T, THandler>(IReadOnly
return false;
}

MemoryHandle handle = handlesToDispose[i] = handler.Pin(in buffer);
long ptr = segments[i] = (long)handle.Pointer;
MemoryHandle handle = handler.Pin(in buffer);
long ptr = (long)handle.Pointer;
if ((ptr & alignedAtPageSizeMask) != 0)
{
handle.Dispose();
return false;
}

// We avoid allocations if there are no
// buffers or the first one is unacceptable.
(handlesToDispose ??= new MemoryHandle[buffersCount])[i] = handle;
if (segments == null)
{
// "The array must contain enough elements to store nNumberOfBytesToWrite
// bytes of data, and one element for the terminating NULL."
segments = (long*)NativeMemory.Alloc((nuint)(buffersCount + 1), sizeof(long));
}
segments[i] = ptr;
}

Debug.Assert(segments != null);
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segments[buffersCount] = 0;
segmentsPtr = (IntPtr)segments;
totalBytes = (int)totalBytes64;
success = true;
return true;
return handlesToDispose != null;
}
finally
{
Expand All @@ -491,14 +504,20 @@ private static unsafe bool TryPrepareScatterGatherBuffers<T, THandler>(IReadOnly
}
}

private static unsafe void CleanupScatterGatherBuffers(MemoryHandle[] handlesToDispose, IntPtr segmentsPtr)
private static unsafe void CleanupScatterGatherBuffers(MemoryHandle[]? handlesToDispose, IntPtr segmentsPtr)
{
foreach (MemoryHandle handle in handlesToDispose)
if (handlesToDispose != null)
{
handle.Dispose();
foreach (MemoryHandle handle in handlesToDispose)
{
handle.Dispose();
}
}

NativeMemory.Free((void*) segmentsPtr);
if (segmentsPtr != IntPtr.Zero)
{
NativeMemory.Free((void*) segmentsPtr);
}
}

private static ValueTask<long> ReadScatterAtOffsetAsync(SafeFileHandle handle, IReadOnlyList<Memory<byte>> buffers,
Expand All @@ -510,7 +529,7 @@ private static ValueTask<long> ReadScatterAtOffsetAsync(SafeFileHandle handle, I
}

if (CanUseScatterGatherWindowsAPIs(handle)
&& TryPrepareScatterGatherBuffers(buffers, default(MemoryHandler), out MemoryHandle[] handlesToDispose, out IntPtr segmentsPtr, out int totalBytes))
&& TryPrepareScatterGatherBuffers(buffers, default(MemoryHandler), out MemoryHandle[]? handlesToDispose, out IntPtr segmentsPtr, out int totalBytes))
{
return ReadScatterAtOffsetSingleSyscallAsync(handle, handlesToDispose, segmentsPtr, fileOffset, totalBytes, cancellationToken);
}
Expand Down Expand Up @@ -607,7 +626,7 @@ private static ValueTask WriteGatherAtOffsetAsync(SafeFileHandle handle, IReadOn
}

if (CanUseScatterGatherWindowsAPIs(handle)
&& TryPrepareScatterGatherBuffers(buffers, default(ReadOnlyMemoryHandler), out MemoryHandle[] handlesToDispose, out IntPtr segmentsPtr, out int totalBytes))
&& TryPrepareScatterGatherBuffers(buffers, default(ReadOnlyMemoryHandler), out MemoryHandle[]? handlesToDispose, out IntPtr segmentsPtr, out int totalBytes))
{
return WriteGatherAtOffsetSingleSyscallAsync(handle, handlesToDispose, segmentsPtr, fileOffset, totalBytes, cancellationToken);
}
Expand Down Expand Up @@ -671,13 +690,12 @@ private static unsafe ValueTask WriteFileGatherAsync(SafeFileHandle handle, IntP

private static async ValueTask WriteGatherAtOffsetMultipleSyscallsAsync(SafeFileHandle handle, IReadOnlyList<ReadOnlyMemory<byte>> buffers, long fileOffset, CancellationToken cancellationToken)
{
long bytesWritten = 0;
int buffersCount = buffers.Count;
for (int i = 0; i < buffersCount; i++)
{
ReadOnlyMemory<byte> rom = buffers[i];
await WriteAtOffsetAsync(handle, rom, fileOffset + bytesWritten, cancellationToken).ConfigureAwait(false);
bytesWritten += rom.Length;
await WriteAtOffsetAsync(handle, rom, fileOffset, cancellationToken).ConfigureAwait(false);
fileOffset += rom.Length;
}
}

Expand Down