Renderer Performance Architecture

Renderer Performance Architecture

Purpose

Design renderer changes so large scenes remain structured, measurable, and scalable. Preserve scene intent first, then choose backend-specific GPU representations deliberately. Performance work has no sacred cows: when telemetry and visual evidence show the current architecture cannot reach the target, replace the architecture instead of optimizing around it.

Workflow

0. Start from the target architecture.

   • Record the architecture class required by the goal before implementing: hardware RT/ray query, resident scene, render graph, temporal accumulation, guided denoising, light tree/reservoir/path guiding, GPU-driven submission, streaming textures, or another proven technique.
   • Use simple paths only as diagnostics, fixtures, compatibility fallbacks, or target-shaped vertical slices. Do not build feature coverage on a path already known to miss the visual, memory, or frame-time target.
   • If multiple measured slices fail because the current pass layout, sampler, denoiser, scene residency, or AS/update model is wrong, plan and execute the larger replacement. Do not keep a slow or visually wrong path as the center of gravity because it is already wired up.

1. Establish the correctness gate.

   • Name the reference renderer or analytic fixture that defines correct output before making a performance claim.
   • Confirm load counts, material/light counts, camera metadata, tone mapping, and key AOVs for the workload.
   • If the scene is physically wrong, classify each issue as a correctness bug, missing feature, quality gap, or performance bottleneck before optimizing.
   • Do not optimize a fast path that omits required geometry, materials, lights, transparency, shadows, or GI without labeling it as degraded.

2. Define the performance claim.

   • Identify the target workload: many instances, many unique meshes, huge textures, many lights, RT shadows, tiled environments, interactive viewing, offline capture, or animation playback.
   • Set budgets for load time, first image, frame time, draw calls, upload bytes, texture memory, acceleration-structure build time, readback time, and peak RSS/VRAM.
   • Record current telemetry before changing architecture.

3. Preserve source structure.

   • Keep USD prototypes, PointInstancers, native instances, material bindings, paths, visibility, transforms, time samples, and payload state distinct during ingest.
   • Build a prototype table and instance table. Do not make a flat mesh array the only internal representation.
   • Treat flattening as an explicit fallback with telemetry, not the default for large scenes.

4. Separate dirty types.

   • Topology changes can rebuild geometry and acceleration structures.
   • Transform, visibility, material parameter, camera, light, render-mode, and time changes should have narrower update paths.
   • Avoid full scene reloads during playback, camera motion, visibility toggles, and capture option changes.

5. Design geometry storage for residency.

   • Use compact vertex layouts and split rarely used attributes into separate streams.
   • Preserve shared prototype buffers and store transforms/material overrides in instance buffers.
   • Use chunked allocation, 64-bit size accounting, explicit overflow checks, and resource-class memory telemetry.
   • Plan for partial rebuilds and streaming before adding large asset test cases.

6. Make uploads asynchronous and batched.

   • Prefer persistent staging rings or reusable upload heaps.
   • Batch copies, texture uploads, mip generation, and buffer updates into fewer command submissions.
   • Do not use queue-wide idle waits or per-resource blocking waits in normal large-scene paths.
   • Surface upload progress and cancellation for long asset operations.

7. Batch draw submission.

   • Group by pipeline, render state, material, vertex layout, prototype, and index range.
   • Use instanced draws for repeated prototypes.
   • Use multi-draw indirect, indirect command buffers, or backend equivalents where available.
   • Store per-instance data in GPU buffers instead of recording CPU push constants for every object.

8. Treat acceleration structures as managed resources.

   • Reuse prototype BLASes across instances.
   • Build all BLAS from one pooled allocation with batched builds; keep memory-allocation and submit counts O(1) / O(n÷batch), not O(n). If the pool allocation fails from VRAM fragmentation, binary-search the size (halve and retry) instead of giving up, and keep a fixed safety margin for driver overhead.
   • When VRAM forces a partial BLAS subset, do not pack smallest-first to maximize count — that omits the few huge BLASes (terrain, roads) that dominate the image. Reserve budget for the largest visible BLASes, then fill the remainder with small meshes.
   • A persistent AS/geometry cache must be content-keyed (a geometry hash), not index-keyed — USD parse order is nondeterministic across cold loads, so an index key misses constantly; memcmp-verify a hash hit, exclude per-instance attributes (transform, color) from the key, and treat a baked-in material id as a separate group key.
   • Build/refit TLASes asynchronously when possible.
   • Partition AS work only when it reduces traversal, rebuild, or memory cost.
   • Report partial RT coverage explicitly; never let raster and RT silently disagree about omitted geometry.
   • Keep primary, shadow, reflection, and tiled/per-env rays consistent about which TLAS they query.
   • Keep alpha cutout, opacity, transmission, two-sidedness, and visibility policy consistent between raster, primary RT, shadow RT, and GI paths.

9. Make materials and textures demand-driven.

   • Cache material bindings by prim path/ancestor chain.
   • Deduplicate textures by resolved identity plus sampler/color-space semantics.
   • Preserve material term semantics while optimizing: base color, data textures, normals, roughness, metallic, opacity, emissive, and texture transforms cannot share an undifferentiated upload path.
   • Apply memory budgets to decoded images, UDIM atlases, mip chains, and GPU texture residency.
   • Upload visible or high-priority textures first and use explicit placeholders for pending assets.

10. Gate expensive passes.

• Use a render graph or pass scheduler for AOVs, shadows, debug views, resolve/copy work, and readback.
• Produce only requested outputs.
• For real-time path tracing with hard HDRI sunlight, many emitters, or noisy GI, separate direct-light reuse, GI reuse/path guiding, and guided denoising into measurable pass classes rather than hiding them inside one monolithic beauty dispatch.
• Prefer clustered, tiled, forward+, or deferred lighting when light count makes simple forward shading expensive.
• Keep debug and validation modes available but out of the normal hot path.
• Keep light sampling tables, PDFs, MIS weights, and pass filters synchronized so performance filtering does not bias transport.

11. Remove readback from interactive rendering.

• Interactive viewing should render without fetching pixels to the CPU.
• Screenshot and golden-image paths should use explicit readiness/fence APIs.
• Use readback rings or backend equivalents and reuse CPU buffers.

12. Validate with scale-aware tests.

• Keep tiny deterministic correctness scenes.
• Add medium scenes that exercise instancing, materials, textures, curves, lights, and transforms.
• Make large showcase scenes opt-in, preflighted, and telemetry-first.
• Record counts, timings, memory, AOV/image metrics, sample settings, and reference-renderer deltas so performance regressions are explainable without hiding correctness regressions.

Review Checklist

• Does the ingest path preserve prototypes and instances until the backend consumes them?
• Is the optimized path still compared against a reference or analytic correctness fixture?
• Does a time/transform/visibility edit avoid full geometry/material reload?
• Are large uploads batched without queue-wide idle waits?
• Are draw calls proportional to batches or visible prototypes, not raw instance count?
• Are textures decoded/uploaded under a budget with deduplication and clear color-space policy?
• Are BLAS/TLAS builds asynchronous or at least isolated from ordinary frame rendering?
• Do primary, shadow, reflection, and GI rays agree on geometry, visibility, opacity, and alpha policy?
• Are light sampling PDFs and MIS weights valid after culling, proxying, or filtering lights?
• Are AOVs, shadows, and readbacks produced only when requested?
• Are hard limits reported as structured errors instead of silent truncation?
• Are memory and timing metrics split by geometry, texture, material, AS, render targets, upload, and readback?
• Is backend parity explicit: supported, degraded, stubbed, or unavailable?

Common Anti-Patterns

• Expanding PointInstancers into full mesh records before renderer import.
• Uploading a second full vertex buffer for a material mode instead of using compact streams.
• Recording one draw call per mesh or instance after already merging geometry.
• Waiting for the whole GPU queue after each staging copy, texture upload, BLAS batch, screenshot, or readback.
• Scanning entire asset directories for every material load.
• Stitching unbounded UDIM atlases without decoded-memory estimates.
• Rebuilding the entire renderer scene for time changes.
• Copying depth, normals, segmentation, or debug buffers when the current request does not need them.
• Letting RT omit geometry silently while raster still draws it.
• Running large golden captures without preflight estimates and incremental telemetry.
• Claiming a speedup while material terms, shadows, transparency, or GI are known to diverge from the reference.
• Extending a known-insufficient fallback instead of moving work onto the required target architecture.
• Treating narrow implementation scope as safer after evidence shows the existing architecture is the risk.

Handoff

When finishing performance architecture work, report:

• Which workload and budget the change targets.
• Which target architecture class the change advances, and which temporary paths remain.
• What scene structure is preserved or intentionally flattened.
• Expected effects on draw count, upload bytes, AS build time, texture memory, readback, and frame time.
• Exact telemetry or tests used to validate the claim.
• Remaining backend-specific gaps and any platform that still needs hand-off validation.