DarkRoom/docs/frame-budget.md

What a frame costs

Status: Measured · 2026-08-27 Companion to: display-and-extension.md §2–3 · requirements.md §3.4 FR-DSP-2, FR-DSP-3, FR-DSP-4 Instrument: core/dr-gpu/examples/frame_budget.rs Guard: core/dr-gpu/tests/frame_budget.rs

display-and-extension.md §2 fixed a decision rule in advance and made three measurements the thing that settles it. This file is those measurements, and the recommendation they support.

Rerun with:

cargo run --release -p dr-gpu --example frame_budget

and diff this file. That is the whole point of committing numbers: a regression should be a diff rather than somebody's recollection of how fast it used to be.

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The answer, first

FR-DSP-2 should be rewritten, not implemented. M1 and M2 sit inside the 16 ms budget at the 99th percentile for every chain of point operations at every viewport size measured, fit and at 1:1 — the widest case, every operation that contributes a fragment to the fused shader at 4K, costs 4.5 ms on the GPU and 8.2 ms including the composition that precedes it. Tiling the interactive path would be optimising something that is already using a quarter of its budget.

But the measurement did find a budget-breaker, and it is not the one tiling fixes. The neighbourhood stage — clarity in particular — costs 34 ms at 4K on its own, twice the whole budget, and tiles do not help it: a tile of a convolution has to read its halo, so tiling raises the total tap count rather than lowering it. §2 predicted this exactly ("a separable blur at a large radius is the plausible budget-breaker, not the fused pass"), and the fix it needs is the one local_contrast's own module documentation already names — a base computed at reduced resolution — which is a change to crate::detail, not a tile scheduler.

There is a third finding nobody was looking for: shader composition costs 3–5 ms of CPU per frame on a full chain, on the UI thread, before any GPU work is submitted. That is a fifth to a third of the budget spent formatting strings, and it is invisible to any amount of tiling.

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Conditions

Adapter	NVIDIA GeForce RTX 3050 6GB Laptop GPU (Vulkan)
Source	9504 × 6336 synthetic (60.2 MP, 482 MB as rgba16f)
Frames	100 measured per row, 12 warm-up frames discarded
Percentile	Nearest-rank, so p99 of 100 frames is the second-worst frame
Build	--release
Date	2026-08-27

shader is EditGraph::compose alone. cpu adds the detail chain and the invalidation hash — everything DevelopSession::render does per frame before it dispatches. gpu is submit plus wait-for-idle, which serialises the GPU work into the frame that caused it and is therefore pessimistic. TOTAL ranks cpu + gpu summed within each frame, which is the column the budget is judged on; adding two percentiles instead would invent a stutter that no frame actually had.

Chains: one is exposure. five is exposure, contrast, highlights/shadows, blacks/whites, vibrance. point is every operation in the default chain that contributes a fragment to the fused shader, film stock included. all is point plus the four neighbourhood operations — noise reduction, capture sharpening, clarity and texture.

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M1 — the fused pass at proxy resolution

The develop view: the whole frame fit to the viewport.

size	chain	shader	cpu p99	gpu p50	gpu p99	TOTAL
1920 × 1200	one	0.08ms	0.10ms	1.02ms	1.23ms	1.31ms
1920 × 1200	five	0.18ms	0.20ms	1.01ms	1.20ms	1.36ms
1920 × 1200	point	2.79ms	2.82ms	1.98ms	2.18ms	4.83ms
1920 × 1200	all	3.65ms	4.73ms	6.86ms	7.37ms	12.02ms
2560 × 1600	one	0.08ms	0.10ms	1.73ms	2.00ms	2.12ms
2560 × 1600	five	0.24ms	0.27ms	1.73ms	2.26ms	2.46ms
2560 × 1600	point	2.82ms	2.85ms	2.37ms	2.65ms	5.38ms
2560 × 1600	all	3.37ms	4.35ms	14.31ms	15.65ms	18.42ms	OVER
3840 × 2160	one	0.10ms	0.14ms	3.09ms	3.31ms	3.42ms
3840 × 2160	five	0.30ms	0.32ms	3.03ms	3.40ms	3.61ms
3840 × 2160	point	3.62ms	3.65ms	4.12ms	4.52ms	8.23ms
3840 × 2160	all	4.12ms	5.07ms	37.73ms	40.17ms	43.24ms	OVER

Read the point rows: the fused dispatch scales with pixels and almost not at all with chain length. Going from one operation to the entire point chain at 4K costs 1.2 ms of GPU. Going from 2.3 M pixels to 8.3 M costs 2.3 ms. Both are small, and the second is the one tiling would address.

The all rows go over, and the point rows in the same block are what say why: the difference between them is the neighbourhood stage, measured on its own in M3 and arriving at almost exactly the same figure.

M2 — the same, zoomed to 1:1 on the 60 MP source

FR-DSP-5's case. Framing::view shrinks the sampled region while the render target keeps its size, so one render pixel lands on one source pixel.

size	chain	shader	cpu p99	gpu p50	gpu p99	TOTAL
1920 × 1200	one	0.12ms	0.14ms	0.42ms	0.66ms	0.75ms
1920 × 1200	five	0.27ms	0.30ms	0.49ms	1.14ms	1.22ms
1920 × 1200	point	3.49ms	3.52ms	1.18ms	1.39ms	4.85ms
1920 × 1200	all	3.64ms	5.18ms	8.96ms	9.55ms	14.30ms
2560 × 1600	one	0.11ms	0.12ms	0.56ms	0.99ms	1.06ms
2560 × 1600	five	0.22ms	0.25ms	0.77ms	1.02ms	1.17ms
2560 × 1600	point	2.96ms	2.99ms	2.03ms	2.52ms	5.61ms
2560 × 1600	all	5.24ms	7.35ms	18.80ms	21.62ms	25.81ms	OVER
3840 × 2160	one	0.10ms	0.12ms	1.31ms	1.52ms	1.63ms
3840 × 2160	five	0.14ms	0.27ms	1.39ms	1.64ms	1.75ms
3840 × 2160	point	3.14ms	3.16ms	4.04ms	4.50ms	7.21ms
3840 × 2160	all	4.84ms	6.78ms	47.22ms	48.79ms	54.47ms	OVER

A 1:1 view of a 60 MP file is cheaper than the fit view of the same file, for every point chain and at every size — 1.52 ms against 3.31 ms for one operation at 4K. That is not a rounding artefact and it is worth stating plainly, because it is the opposite of what "full resolution" sounds like it should cost. The dispatch is the same number of pixels either way; what changes is where those pixels read from. A fit view walks the whole 482 MB texture on a stride, and a 1:1 view reads a contiguous window of it that fits comfortably in cache.

So the resolution FR-DSP-5 promises costs nothing extra on the fused path. Zooming is not an expensive mode to be dreaded and progressively refined into; it is the cheap one.

The all rows are worse at 1:1 than fit, and that is the detail stage again for a specific reason: noise reduction's radius is stated in source pixels, so RenderScale::ratio climbing to 1.0 widens its kernel. Clarity's is stated as a fraction of the frame and does not move. M3 separates the two.

M3 — the neighbourhood stage alone

Timed with the fused dispatch deliberately reused: only a detail parameter moves, so render_detailed skips the colour pass (FR-DEV-3d) and what remains is the convolutions. colour counts fused dispatches over the measured frames and is zero on every row, which is what makes these numbers mean "detail alone" rather than asserting it.

size	stage	view	pass	radius	colour	cpu p99	p50	p99
1920 × 1200	clarity	fit	2	29	0	1.60ms	5.42ms	5.99ms
1920 × 1200	all four	fit	7	29	0	1.71ms	5.78ms	6.16ms
1920 × 1200	clarity	1:1	2	29	0	1.12ms	7.51ms	8.01ms
1920 × 1200	all four	1:1	9	29	0	2.71ms	8.25ms	9.11ms
2560 × 1600	clarity	fit	2	38	0	1.03ms	12.02ms	12.44ms
2560 × 1600	all four	fit	7	38	0	1.87ms	12.49ms	13.16ms
2560 × 1600	clarity	1:1	2	38	0	1.87ms	15.82ms	16.60ms
2560 × 1600	all four	1:1	9	38	0	2.71ms	17.24ms	18.06ms
3840 × 2160	clarity	fit	2	52	0	1.75ms	33.11ms	33.89ms
3840 × 2160	all four	fit	7	52	0	1.76ms	34.21ms	35.03ms
3840 × 2160	clarity	1:1	2	52	0	1.08ms	40.39ms	41.86ms
3840 × 2160	all four	1:1	9	52	0	2.37ms	43.29ms	44.72ms

radius is the widest halo any pass reads, in render pixels.

Clarity alone is 97% of the cost of all four neighbourhood operations together, at every size. Its σ is 1.2% of the shorter edge and it truncates at 2σ, so its radius is 29 px on a 1200 px viewport and 52 px at 4K — two separable passes of 105 taps each, over 8.3 M pixels, which is 1.7 billion texture reads. That is the whole of the problem, and the numbers scale as radius × pixels exactly as that description predicts: 5.99 → 12.44 → 33.89 ms for radii of 29 → 38 → 52 over 2.3 → 4.1 → 8.3 M pixels.

The extra cost at 1:1 is noise reduction and capture sharpening, whose radii are properties of the sensor rather than of the frame. That is the correct behaviour — it is why RenderScale has two units — and it is bounded by the kernel caps those operations already declare.

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Reading this against §2's decision rule

§2: "If M1 and M2 sit inside 16 ms at the 99th percentile, FR-DSP-2 is rewritten rather than implemented … If they do not, the measurement tells us which stage to tile."

Both halves of the rule fire, on different stages, and the honest reading takes both.

FR-DSP-2 — rewrite it

For the fused pass the rule passes with a wide margin. Every point chain at every size, fit and at 1:1, is inside 16 ms — the worst TOTAL is 8.23 ms and the worst GPU figure is 4.52 ms. There is no viewport size on a desktop display where recomputing the entire point chain over every visible pixel is a problem.

Two further reasons not to build the tile scheduler as written:

1. Panning, which is the case ARCH §5.3's tile cache is designed for, gets no benefit here. Reusing already-valid tiles saves recomputation. Recomputing the whole 4K viewport costs 4.5 ms, so a perfect tile cache could save at most 4.5 ms of a 16 ms budget, at the price of a cache keyed by (VersionId, tile, zoom, graph_hash_prefix) that has to stay correct across every parameter change in the graph. That is a large correctness surface bought with a small number.

2. It would make the actual problem worse. The stage that misses the budget is a convolution, and a tiled convolution reads a halo per tile. At a 52-pixel radius, 256-pixel tiles would read (256+104)² instead of 256² — very nearly twice the taps. Tiling is the wrong tool for the one stage that needs a tool.

So FR-DSP-2 becomes what §2 said it actually is for this architecture: a scheduling concern for export and thumbnailing, both of which already run off the frame path. The interactive path does not tile.

The stage that does need work — and it is not tiling

The measurement's real product is naming the stage. It is local_contrast, and the fix is stated in that module's own documentation:

The right optimisation is a base computed at reduced resolution, which needs a detail stage that can write a smaller target than it reads; that is a change to crate::detail, not to this file.

A Gaussian base at a quarter resolution is 1/16 the pixels at 1/4 the radius — about 1/64 of the work — and the result is visually identical because a base at σ = 26 px has no content above the quarter-resolution Nyquist to lose. That is a change to two files with a bounded blast radius, and it is what the 34 ms buys back. It should be tracked as its own item rather than smuggled in under a requirement about tiles.

FR-DSP-3 — the clause that should be narrowed

§3.3 proposes narrowing "when a full-resolution result is needed it is computed asynchronously, and the proxy result remains on screen until it is ready" to export and 1:1 zoom, or striking it.

M2 says strike it. The clause exists to hide the latency of a full-resolution render behind a proxy. There is no such latency: the 1:1 view is faster than the fit view on the fused path, and there is no second full-resolution code path to be asynchronous about — Framing::view is the whole mechanism. Export renders its own frames on a worker already. Keeping the clause would mean building a progressive-swap machine to conceal a render that completes in 1.4 ms.

FR-DSP-4 — satisfied vacuously, on the fused path

§4 makes progressive refinement conditional on M1 failing. On the fused path M1 passes, so reduced-quality rendering during a drag would buy nothing and cost the visible softness the requirement itself warns against.

The neighbourhood stage is the exception, and it is worth being precise: what that stage needs is not progressive refinement — it is a permanently cheaper base, computed at reduced resolution and correct at any moment the user stops. "Render coarse while dragging, sharpen when it settles" would paper over the same 34 ms with a visible swap. Fix the stage.

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Which GPU, on a machine with more than one

Measured 2026-08-29 on a laptop holding an Intel Iris Xe (RPL-P) and an AMD RX 5700 XT, same binary, adapter forced with VK_ICD_FILENAMES.

The question was whether an integrated GPU is the better choice for this application. The argument for it is good: a 24 MP frame is ~96 MB of RGBA, and on a discrete card every upload and every export readback crosses PCIe, where an iGPU shares memory with the CPU and crosses nothing. It also does not empty a battery.

The compute says otherwise, and not marginally.

2560×1600, p99	AMD RX 5700 XT	Intel Iris Xe
fused pass, point	5.19 ms	7.75 ms
fused pass, all	11.70 ms	66.42 ms
M3 clarity, fit	4.67 ms	38.28 ms
M3 all four, 1:1	6.44 ms	57.67 ms

1920×1200, M3 clarity, fit	2.35 ms	19.99 ms

The fused colour pass is within a factor of 1.5 — it is one read and one write per pixel, which an iGPU does perfectly well. The neighbourhood stage is 5–8× slower, and that is what decides it: clarity at 1920×1200 costs 20 ms on the Iris Xe, so it leaves the budget on its own at the smallest size tested, before anything else in the chain runs.

So the default adapter preference stays Performance (dr_gpu::AdapterPreference).

Two things this does not show, and neither is a reason to revisit the default without measuring them:

• It does not refute the transfer argument. This harness renders from a resident texture and never uploads or reads back, so the PCIe cost an iGPU avoids does not appear in any column above. Import, export and the thumbnail sweeps are transfer-heavy and compute-trivial, and may well go the other way — but they are not what FR-DSP-3 bounds, and one device is opened at startup and shared with the compositor, so there is currently no way to use a different adapter for a different task.
• It says nothing about power. Efficiency remains offered (DARKROOM_GPU=integrated) because a user on battery may rationally accept a slower detail chain, and because someone whose discrete card has failed needs a way to keep working.

What is not measured here

Stated because §7 of display-and-extension.md asks for it, and because each of these could move the numbers.

• Local adjustments. The mask stack is a separate chain per layer and is not in any row above. render_masked takes them and the fused shader addresses them per layer, so a heavily masked edit costs more than all.
• Spot repairs. These add detail passes, and their cost is per spot.
• Lens corrections. Not part of EditGraph::default_chain — they are built from a matched profile — so the point row does not include the warp chain.
• Demosaic. Once per photograph on a worker, not on the frame path.
• Presentation. The bench waits for the device to go idle inside the frame it measures. A real compositor overlaps frames, so these figures are an upper bound rather than an estimate.
• One adapter. A discrete laptop GPU. The Intel iGPU on the same machine, and Android, will be slower — which is an argument for the conclusion rather than against it: the stage with no headroom has none to lose.

The CPU finding, which deserves its own item

EditGraph::compose costs 2.8–5.2 ms per frame on a full chain, at every resolution, because it is resolution-independent: it assembles a WGSL string and hashes it. On the all rows it is a third of what is left of the budget after the GPU has taken its share, and at 1920 × 1200 it is larger than the entire fused dispatch.

Nothing in this document's recommendations changes it, and it is the cheapest remaining win. The generated source depends only on the structure of the graph — that is what structure_hash already identifies, and it is precisely what does not change while a slider is being dragged, which is why the pipeline cache in AdjustPass does not recompile. The uniforms do change, but assembling them is a handful of floats per operation. So caching the source string against the structure hash and rebuilding only the uniforms would take these milliseconds to approximately nothing, on the path that needs them most. Worth its own entry in technical-debt.md.