Apply Inference Optimizations

Apply inference optimizations

Use this skill after profile-model-performance has identified the dominant stage. Keep every optimization opt-in until benchmark and quality evidence show that it is safe for the target workflow.

Ground rules

• Preserve the existing quality path as the baseline. Add faster paths as flags or config variants first.
• Change one optimization family at a time unless a combined profile is the explicit validation target.
• Add or extend the benchmark harness in the same change as the runtime flag.
• After applying an optimization, continue with validate-performance-quality before promoting it as a default or documenting a speedup claim.
• Print active settings and stage timing summaries so runs are auditable.
• Read flashdreams-integrations before moving code across core, infra, recipes, or integrations. Avoid model-specific branches in shared layers; add config slots or override hooks instead.

Model and denoise path

Use these when the measured hot stage is the transformer, scheduler loop, or model-side cache interaction.

• Prefer fixed shapes before compiler work: bounded history windows, static chunk sizes, preallocated buffers, and stable cache storage.
• Restrict torch.compile to the smallest fixed-shape callable that contains real compute. Keep cache mutation, dynamic setup, reset, and I/O outside the compiled region unless a probe proves the broader scope is worth it.
• Separate pre-saturation and post-saturation behavior. Dynamic cache fill can remain eager while the saturated steady-state call is compiled.
• Test compile modes and persistent compiler cache behavior in fresh processes. Record first-visible chunk cost, hidden prewarm cost, and steady-state gain.
• Add CUDA graph capture only around a region with stable shapes, stable device pointers, deterministic stream ordering, and explicit warmup. Start with the smallest useful graph before attempting whole-step capture.
• Profile actual SDPA kernels before forcing attention backends. If the default already dispatches to the desired backend, backend forcing is unlikely to help and may change numerics.

Cache path

Use these when timing shows append/slice churn, K/V refresh, history-window growth, or cache synchronization cost.

• Replace repeated concatenate/slice pruning with fixed-size block storage when the attention semantics allow it. Include sink tokens or pinned context regions only if the original model relied on them.
• Prove parity before and after the rolling boundary. Short pre-roll equality is not enough if the bug only appears once the window evicts old frames.
• Keep finalized history and current in-flight tokens equivalent to the baseline path; off-by-one cache windows create plausible but divergent rollouts.
• Overlap cache maintenance on a side stream only after the synchronous path is correct. Report both submit time and next-step wait time.
• Reset, scene switches, prompt changes, and shape changes must cancel or flush pending async cache work and rebuild state deliberately.

Decode path

Use these when VAE or decoder time dominates the chunk budget.

• Keep the full-quality decoder as the reference path unless the user explicitly chooses a preview-quality mode.
• Build a decoder-only probe that decodes identical latents through reference and candidate paths. Do not use separate autoregressive rollouts as strict decoder-quality evidence.
• Try layout and memory-format changes before broad compiler changes when profiling shows copy/layout overhead.
• Compile only stateless or carefully isolated decoder submodules first. Whole streaming decoder compilation can silently corrupt cache state or alter numerics; reject it unless every output frame matches the reference within the accepted tolerance.
• Treat CUDA graph without compile as a correctness probe first. If it only saves a few milliseconds, record that and avoid live-path complexity.
• Lightweight decoders are preview candidates until same-latent metrics and visual artifacts show they are close enough to the quality decoder for the intended use.

Transfer and presentation path

Use these when generated frames are ready faster than users see them, or when CPU work dominates after decode.

• Delay GPU-to-CPU materialization until the consumer needs frames. Use lazy CUDA frame objects, pinned host prefetch, or batched copies when the local code already has those patterns.
• Measure CPU image/video encode before moving it to a thread or GPU codec.
• Preserve generated frame order for quality demos. Frame dropping can diagnose queue backlog, but it changes motion continuity and should not be the recommended quality path.
• Tune ordered pacing and bounded queues after throughput changes. Faster generation can feel slower if old frames accumulate in the presenter.
• Report best, average, and worst estimated input-to-visible latency when the workflow is interactive.

Promotion criteria

Promote an optimization to the Recommended validation status only when all apply:

• steady-state total chunk time or target latency improves materially;
• stage timing shows the expected bottleneck moved or shrank;
• quality validation passes against the right reference;
• startup, prewarm, reset, scene-switch, and shape-change behavior are acceptable;
• the fallback path remains available;
• docs state the command, caveats, whether the setting is enabled by default, and its validate-performance-quality status: Recommended, Useful opt-in, Rejected, or Deferred.

Rejection notes

Record failed attempts with the same care as successful ones: exact command, observed speed, quality result, failure mode, and what would have to change to revisit the idea. This prevents future integrations from repeating unsafe compiler, decoder, cache, or presentation shortcuts.