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[fix] Enable adaptive downscale after client timeouts #291

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Merged
paul-fresquet merged 2 commits into from
Apr 26, 2026
Merged

[fix] Enable adaptive downscale after client timeouts #291

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297ffc3
[fix] Enable adaptive downscale after client timeouts
paul-fresquet Apr 25, 2026
f774b39
[fix] Address adaptive timeout review comments
paul-fresquet Apr 26, 2026
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96 changes: 65 additions & 31 deletions src/ByteSync.Client/Services/Communications/Transfers/Uploading/AdaptiveUploadController.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@
private const int MAX_CHUNK_SIZE_BYTES = 16 * 1024 * 1024; // 16 MB
private const int MIN_PARALLELISM = 2;
private const int MAX_PARALLELISM = 4;
private const int CLIENT_TIMEOUTS_BEFORE_DOWNSCALE = 2;

private const double MULTIPLIER_2_X = 2.0;
private const double MULTIPLIER_1_75_X = 1.75;
Expand All @@ -36,6 +37,7 @@
private readonly Queue<long> _recentBytes;
private int _successesInWindow;
private int _windowSize;
private int _consecutiveClientTimeouts;
private readonly ILogger<AdaptiveUploadController> _logger;
private readonly object _syncRoot = new();

Expand Down Expand Up @@ -86,11 +88,20 @@
{
lock (_syncRoot)
{
if (IsClientSideFailure(uploadResult.FailureKind))
if (uploadResult.FailureKind == UploadFailureKind.ClientCancellation)
{
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_consecutiveClientTimeouts = 0;
return;
}

if (uploadResult.FailureKind == UploadFailureKind.ClientTimeout)
{
HandleClientTimeout(uploadResult.FileId);
return;
}

_consecutiveClientTimeouts = 0;

EnqueueSample(uploadResult.Elapsed, uploadResult.IsSuccess, uploadResult.ActualBytes);

if (HandleBandwidthReset(uploadResult.IsSuccess, uploadResult.StatusCode))
Expand All @@ -105,13 +116,13 @@

var maxElapsed = GetMaxElapsedInWindow();

_logger.LogDebug(
"Adaptive: file {FileId} maxElapsed={MaxElapsedMs} ms, window={Window}, parallelism={Parallelism}, chunkSize={ChunkKb} KB",
uploadResult.FileId ?? "-",
maxElapsed.TotalMilliseconds,
_windowSize,
_currentParallelism,
Math.Round(_currentChunkSizeBytes / 1024d));

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if (TryHandleDownscale(maxElapsed, uploadResult.FileId))
{
Expand Down Expand Up @@ -151,9 +162,26 @@
}
}

private static bool IsClientSideFailure(UploadFailureKind failureKind)
private void HandleClientTimeout(string? fileId)
{
return failureKind is UploadFailureKind.ClientCancellation or UploadFailureKind.ClientTimeout;
_consecutiveClientTimeouts += 1;
if (_consecutiveClientTimeouts < CLIENT_TIMEOUTS_BEFORE_DOWNSCALE)
{
_logger.LogDebug(
"Adaptive: file {FileId} client timeout {TimeoutCount}/{Threshold}. Waiting before downscale",
fileId ?? "-",
_consecutiveClientTimeouts,
CLIENT_TIMEOUTS_BEFORE_DOWNSCALE);

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return;
}

_logger.LogInformation(
"Adaptive: file {FileId} client timeout threshold reached ({TimeoutCount}). Downscaling upload settings",
fileId ?? "-",
_consecutiveClientTimeouts);

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_consecutiveClientTimeouts = 0;
Downscale(fileId, "client timeouts");
}

private bool HandleBandwidthReset(bool isSuccess, int? statusCode)
Expand Down Expand Up @@ -192,40 +220,45 @@
{
if (maxElapsed > _downscaleThreshold)
{
if (_currentParallelism > MIN_PARALLELISM)
{
_logger.LogInformation(
"Adaptive: file {FileId} Downscale. Reducing parallelism {Prev} -> {Next}. Resetting window (window before {WindowBefore})",
fileId ?? "-",
_currentParallelism, _currentParallelism - 1,
_windowSize);
_currentParallelism -= 1;
_windowSize = _currentParallelism;
ResetWindow();

return true;
}

var reduced = (int)Math.Max(MIN_CHUNK_SIZE_BYTES, _currentChunkSizeBytes * 0.75);
if (reduced != _currentChunkSizeBytes)
{
_currentChunkSizeBytes = reduced;
_logger.LogInformation(
"Adaptive: file {FileId} Downscale. maxElapsed={MaxElapsedMs} ms > {ThresholdMs} ms. New chunkSize={ChunkKb} KB",
fileId ?? "-",
maxElapsed.TotalMilliseconds,
_downscaleThreshold.TotalMilliseconds,
Math.Round(_currentChunkSizeBytes / 1024d));
}

ResetWindow();

Downscale(fileId, $"maxElapsed={maxElapsed.TotalMilliseconds} ms > {_downscaleThreshold.TotalMilliseconds} ms");
return true;
}

return false;
}

private void Downscale(string? fileId, string reason)
{
if (_currentParallelism > MIN_PARALLELISM)
{
_logger.LogInformation(
"Adaptive: file {FileId} Downscale ({Reason}). Reducing parallelism {Prev} -> {Next}. Resetting window (window before {WindowBefore})",
fileId ?? "-",
reason,
_currentParallelism,
_currentParallelism - 1,
_windowSize);

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_currentParallelism -= 1;
_windowSize = _currentParallelism;
ResetWindow();

return;
}

var reduced = (int)Math.Max(MIN_CHUNK_SIZE_BYTES, _currentChunkSizeBytes * 0.75);
if (reduced != _currentChunkSizeBytes)
{
_currentChunkSizeBytes = reduced;
_logger.LogInformation(
"Adaptive: file {FileId} Downscale ({Reason}). New chunkSize={ChunkKb} KB",
fileId ?? "-",
reason,
Math.Round(_currentChunkSizeBytes / 1024d));

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}

ResetWindow();
}

private void TryHandleUpscale(string? fileId)
{
var recentDurations = _recentDurations.ToArray();
Expand Down Expand Up @@ -273,12 +306,12 @@
var increased = (int)Math.Round(_currentChunkSizeBytes * multiplier);
_currentChunkSizeBytes = Math.Clamp(increased, MIN_CHUNK_SIZE_BYTES, MAX_CHUNK_SIZE_BYTES);

_logger.LogInformation(
"Adaptive: file {FileId} Upscale. maxElapsed={MaxElapsedMs} ms <= {ThresholdMs} ms. New chunkSize={ChunkKb} KB",
fileId ?? "-",
maxElapsedEligible.TotalMilliseconds,
_upscaleThreshold.TotalMilliseconds,
Math.Round(_currentChunkSizeBytes / 1024d));

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UpdateParallelismOnUpscale(fileId);
_currentParallelism = Math.Min(_currentParallelism, MAX_PARALLELISM);
Expand Down Expand Up @@ -316,8 +349,8 @@
_currentParallelism = Math.Max(_currentParallelism, 4);
if (_currentParallelism != prev)
{
_logger.LogInformation("Adaptive: file {FileId} Upscale. Increasing parallelism {Prev} -> {Next} due to chunk>=8MB",
fileId ?? "-", prev, _currentParallelism);

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}
}
else if (_currentChunkSizeBytes >= FOUR_MB)
Expand All @@ -326,8 +359,8 @@
_currentParallelism = Math.Max(_currentParallelism, 3);
if (_currentParallelism != prev)
{
_logger.LogInformation("Adaptive: file {FileId} Upscale. Increasing parallelism {Prev} -> {Next} due to chunk>=4MB",
fileId ?? "-", prev, _currentParallelism);

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}
}
}
Expand Down Expand Up @@ -362,6 +395,7 @@
_currentChunkSizeBytes = Math.Clamp(INITIAL_CHUNK_SIZE_BYTES, MIN_CHUNK_SIZE_BYTES, MAX_CHUNK_SIZE_BYTES);
_currentParallelism = MIN_PARALLELISM;
_windowSize = _currentParallelism;
_consecutiveClientTimeouts = 0;
}

ResetWindow();
Expand Down
83 changes: 58 additions & 25 deletions ...nt.UnitTests/Services/Communications/Transfers/Uploading/AdaptiveUploadControllerTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -93,6 +93,7 @@ public void Downscale_ReducesParallelism_WhenAboveMin()
FeedFastWindow(_controller);
}

_controller.CurrentChunkSizeBytes.Should().BeGreaterThanOrEqualTo(4 * 1024 * 1024);
_controller.CurrentParallelism.Should().BeGreaterThan(2);
var beforeParallelism = _controller.CurrentParallelism;
var beforeChunk = _controller.CurrentChunkSizeBytes;
Expand Down Expand Up @@ -184,40 +185,58 @@ public void ClientTimeout_DoesNotResetChunkSize()
}

[Test]
public void ClientTimeout_DoesNotEnterAdaptiveWindow_AndDoesNotResetChunkSize()
public void ClientTimeouts_DownscaleBelowInitialChunkSize_WhenAtMinParallelism()
{
// Arrange
_controller.CurrentParallelism.Should().Be(2);
_controller.CurrentChunkSizeBytes.Should().Be(500 * 1024);

// Act - Two consecutive client-side timeouts trigger controlled downscale
FeedClientTimeouts(_controller, 2);

// Assert
_controller.CurrentParallelism.Should().Be(2);
_controller.CurrentChunkSizeBytes.Should().BeLessThan(500 * 1024);
_controller.CurrentChunkSizeBytes.Should().Be(375 * 1024);
}

[Test]
public void ClientTimeouts_ReduceParallelismFirst_WhenAboveMinParallelism()
{
// Arrange - Inflate chunk and make sure parallelism is just at min (=2)
var safety = 10;
while (_controller.CurrentChunkSizeBytes < 1024 * 1024 && safety-- > 0)
// Arrange
var safety = 100;
while (_controller.CurrentChunkSizeBytes < 4 * 1024 * 1024 && safety-- > 0)
{
FeedFastWindow(_controller);
}

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_controller.CurrentParallelism.Should().BeGreaterThan(2);
var beforeParallelism = _controller.CurrentParallelism;
var beforeChunk = _controller.CurrentChunkSizeBytes;

// Act
FeedClientTimeouts(_controller, 2);

// Assert
_controller.CurrentParallelism.Should().Be(beforeParallelism - 1);
_controller.CurrentChunkSizeBytes.Should().Be(beforeChunk);
}

[Test]
public void ClientTimeouts_DoNotReduceChunkSizeBelowMinimum()
{
// Arrange
_controller.CurrentParallelism.Should().Be(2);
var inflatedChunk = _controller.CurrentChunkSizeBytes;

// Act - Feed a window of slow client-timeout failures (with the new failure kind)
var p = _controller.CurrentParallelism;
for (var i = 0; i < p; i++)
// Act
for (var i = 0; i < 20; i++)
{
RecordUploadResult(
_controller,
TimeSpan.FromSeconds(60),
isSuccess: false,
partNumber: i + 1,
statusCode: 0,
failureKind: UploadFailureKind.ClientTimeout);
FeedClientTimeouts(_controller, 2);
}

RecordUploadResult(
_controller,
TimeSpan.FromSeconds(1),
isSuccess: true,
partNumber: 100);

// Assert - the timeout samples were ignored and cannot trigger a later downscale
_controller.CurrentChunkSizeBytes.Should().NotBe(500 * 1024);
_controller.CurrentChunkSizeBytes.Should().Be(inflatedChunk,
because: "client-side cancellations are not bandwidth signals and must not influence chunk sizing");
// Assert
_controller.CurrentChunkSizeBytes.Should().Be(64 * 1024);
_controller.CurrentParallelism.Should().Be(2);
}

[Test]
Expand Down Expand Up @@ -248,6 +267,20 @@ private static void FeedWindow(AdaptiveUploadController controller, TimeSpan ela
}
}

private static void FeedClientTimeouts(AdaptiveUploadController controller, int count)
{
for (var i = 0; i < count; i++)
{
RecordUploadResult(
controller,
TimeSpan.FromSeconds(60),
isSuccess: false,
partNumber: i + 1,
statusCode: 0,
failureKind: UploadFailureKind.ClientTimeout);
}
}

private static void RecordUploadResult(
AdaptiveUploadController controller,
TimeSpan elapsed,
Expand Down
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