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perf(capture): overlap GPU readbacks with monotonic publish gate (slice 17)

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Measure take ty-1824 on the slice-16 build: the downscale fix worked (conv
~47ms, ring allocs 0) but desktop band was still 88% frozen at 6.8 updates/s.
Telemetry isolated the real wall — CreateCopyFromSurfaceAsync readback ~45ms
of each conversion, serialized one-in-flight => ~17/s capture cap. Docs fact:
pool-sized surfaces CLIP, not scale (Microsoft Learn), so readback stays
native; the lever is concurrency.

- MaxConcurrentConversions=3 with pool 2->5 buffers (in-flight frames fit)
- new MonotonicGate (Interlocked compare-exchange): stale OLDER completions
  are dropped, never overwrite a newer LatestFrame (mirror of 1742 tear)
- FrameRingBuffer.Rent/ConsumeAllocations now lock; downscale row scratch is
  per-conversion locals
- Good Dog test PublishGate_TryPublish_OnlyStrictlyNewerWins; 296/296 green,
  0 warnings; docs cited Microsoft screen-capture page + libyuv fixed-point.

Local only, no push.
This commit is contained in:
Llama Chile Shop
2026-09-14 18:40:20 -07:00
parent 71932b9756
commit b37b8a30f9
7 changed files with 238 additions and 114 deletions
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Services/ScreenCaptureFrameSource.cs
+40 -25
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@@ -2,6 +2,7 @@ using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Runtime.InteropServices.WindowsRuntime;
using System.Threading;
using System.Threading.Tasks;
using Windows.Graphics;
using Windows.Graphics.Capture;
@@ -28,7 +29,7 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
private GraphicsCaptureSession? _session;
private SizeInt32 _poolSize;
private bool _started;
private bool _framePending;
private int _convertInFlight;
private DateTime _lastErrorLog = DateTime.MinValue;
// Buffer recycling (take-11 spikes, 2026-09-04; slice 16, 2026-09-14): a fresh
@@ -43,8 +44,7 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
// consumers (the compositor's paste cache) cannot false-hit a recycled array.
private readonly FrameRingBuffer _ring = new(8, redLine: 4);
private long _epoch;
private byte[]? _row0;
private byte[]? _row1;
private readonly MonotonicGate _publishGate = new();
// The composition master frame (see ai.md "Resolution tiers"): the background
// is an input layer, so we never hold a CPU frame bigger than the master.
@@ -54,14 +54,19 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
// A failing conversion must not re-flood the log at frame rate.
private static readonly TimeSpan ErrorLogThrottle = TimeSpan.FromSeconds(5);
// Conversion cadence (slice 16, 2026-09-14): the monitor delivers at the 240Hz DWM
// cadence (~4.2ms) while slots are 16.6ms. A 10ms floor between conversion starts
// keeps the open edge above the ~60 conversions/s the pump can actually use, so the
// 240Hz tail stops chewing a conversion thread that the 1742 take measured at
// ~150ms/frame (a 90%-frozen desktop, ~6 fresh frames/s). Drop counters and the
// rolling conversion stats feed the 2-second startup.log telemetry line.
// Conversion cadence (slice 16, 2026-09-14; slice 17, 2026-09-14): the monitor
// delivers at the 240Hz DWM cadence (~4.2ms) while slots are 16.6ms. A 10ms floor
// between conversion starts keeps the open edge above the ~60 conversions/s the pump
// can use. What actually bounded the desktop feed on takes 1742/1824 was the SERIAL
// GPU→CPU readback (`CreateCopyFromSurfaceAsync` ≈ 40-50ms of the ~47ms conversion
// on a 240Hz-HDR box shared with the encoder), capping captures at ~17-20/s.
// Slice 17 overlaps up to MaxConcurrentConversions readbacks (pool sized to
// accommodate in-flight frames) and publishes only monotonically newer frames
// (MonotonicGate — a slow older completion must never overwrite a newer LatestFrame).
private static readonly TimeSpan MinConvertInterval = TimeSpan.FromMilliseconds(10);
private static readonly TimeSpan TelemetryInterval = TimeSpan.FromSeconds(2);
private const int MaxConcurrentConversions = 3;
private const int PoolBufferCount = 5;
private DateTime _lastConvertAt = DateTime.MinValue;
private DateTime _telemetryFrom = DateTime.UtcNow;
private DateTime _lastTelemetry = DateTime.UtcNow;
@@ -107,7 +112,7 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
var item = _item;
var device = Direct3D11Helper.CreateDevice();
var framePool = Direct3D11CaptureFramePool.CreateFreeThreaded(
device, DirectXPixelFormat.B8G8R8A8UIntNormalized, 2, item.Size);
device, DirectXPixelFormat.B8G8R8A8UIntNormalized, PoolBufferCount, item.Size);
_poolSize = item.Size;
var session = framePool.CreateCaptureSession(item);
framePool.FrameArrived += OnFrameArrived;
@@ -124,7 +129,7 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
lock (_gate)
{
_started = false;
_framePending = false;
_convertInFlight = 0;
if (_framePool != null)
_framePool.FrameArrived -= OnFrameArrived;
_session?.Dispose();
@@ -151,14 +156,15 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
if (frame.ContentSize.Width != _poolSize.Width || frame.ContentSize.Height != _poolSize.Height)
{
sender.Recreate(Direct3D11Helper.CreateDevice(),
DirectXPixelFormat.B8G8R8A8UIntNormalized, 2, frame.ContentSize);
DirectXPixelFormat.B8G8R8A8UIntNormalized, PoolBufferCount, frame.ContentSize);
_poolSize = frame.ContentSize;
}
// One conversion at a time, spaced by MinConvertInterval (slice 16): the
// 240Hz delivery otherwise queued a conversion every ~4.2ms and the
// 1742 take's ~150ms conversion pinned the desktop layer ~90% frozen.
if (_framePending)
// Up to MaxConcurrentConversions readbacks in flight (slice 17), spaced by
// MinConvertInterval (slice 16): the 240Hz delivery otherwise queued one
// conversion every ~4.2ms, and the serial ~47ms readback pinned the desktop
// layer to ~17 updates/s on the 1824 take.
if (_convertInFlight >= MaxConcurrentConversions)
{
_skippedBusy++;
frame.Dispose();
@@ -172,7 +178,7 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
return;
}
_lastConvertAt = now;
_framePending = true;
_convertInFlight++;
}
_ = ProcessFrameAsync(frame);
}
@@ -186,7 +192,14 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
using (var softwareBitmap = await SoftwareBitmap.CreateCopyFromSurfaceAsync(
frame.Surface, BitmapAlphaMode.Ignore))
{
FrameAvailable?.Invoke(CopyToVideoFrame(softwareBitmap));
// Monotonic sequencing across the overlapping conversions (slice 17): a
// completed readback is published only if strictly newer than the last
// one published — a slow older completion must never overwrite a newer
// LatestFrame (that would be a time hole in the other direction).
var epoch = Interlocked.Increment(ref _epoch);
var videoFrame = CopyToVideoFrame(softwareBitmap, epoch);
if (_publishGate.TryPublish(epoch))
FrameAvailable?.Invoke(videoFrame);
}
}
catch (Exception ex)
@@ -203,8 +216,8 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
lock (_gate)
{
EmitTelemetry(sw.ElapsedMilliseconds);
_convertInFlight--;
}
_framePending = false;
}
}
@@ -234,7 +247,7 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
_skippedCadence = 0;
}
private VideoFrame CopyToVideoFrame(SoftwareBitmap bitmap)
private VideoFrame CopyToVideoFrame(SoftwareBitmap bitmap, long epoch)
{
var sw = bitmap.PixelWidth;
var sh = bitmap.PixelHeight;
@@ -257,12 +270,12 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
// DWM delivers an opaque surface (alpha 255); bilinear keeps it 255.
var scaled = _ring.Rent(dw * dh * 4);
return new VideoFrame(dw, dh, DownscaleBgra(data, sw, sh, srcStride, dw, dh, scaled))
{ IsOpaque = true, Epoch = ++_epoch };
{ IsOpaque = true, Epoch = epoch };
}
var pixels = _ring.Rent(count);
Marshal.Copy(data, pixels, 0, pixels.Length);
return new VideoFrame(sw, sh, pixels) { IsOpaque = true, Epoch = ++_epoch };
return new VideoFrame(sw, sh, pixels) { IsOpaque = true, Epoch = epoch };
}
// Integer 8.8 fixed-point bilinear downscale to the master frame (slice 16,
@@ -275,9 +288,11 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
// row pair through Marshal.Copy (no unsafe), writing tightly packed BGRA output.
private byte[] DownscaleBgra(IntPtr src, int sw, int sh, int srcStride, int dw, int dh, byte[] dst)
{
// Row scratch is per-capture-thread and reused across frames (same churn lesson).
var row0 = _row0 != null && _row0.Length >= srcStride ? _row0 : (_row0 = new byte[srcStride]);
var row1 = _row1 != null && _row1.Length >= srcStride ? _row1 : (_row1 = new byte[srcStride]);
// Row scratch is per-conversion (overlapping conversions since slice 17 each
// bring their own — two 10KB arrays, no shared state). Size: a 2560-wide row
// pair, the largest the monitor path delivers before the downscale.
var row0 = new byte[srcStride];
var row1 = new byte[srcStride];
for (var y = 0; y < dh; y++)
{