Tune on-device inference performance

Treat tuning as a measurement problem:

1. Pick a representative prompt or workload.
2. Record baseline timings.
3. Change one variable at a time.
4. Keep the fastest stable configuration.

Pick a tuning goal#

• First-token latency: tune load-time setup, prompt size and prompt evaluation cost.
• Sustained throughput: tune backend choice, the decode path and batching.
• Stability: lower GPU pressure, reduce context and keep multimodal inputs small.
• Multimodal responsiveness: reduce image and audio size first, then revisit backend and token budget.

If you do not know which goal matters most, start with latency and stability.

Work in this order:

1. Benchmark the exact prompt shape you care about.
2. Compare cpu and GPU backends before changing anything else.
3. Reduce contextSize and maxTokens to the smallest values that fit your use case.
4. Tune load-time knobs (gpuLayers, threads, batch sizes) one at a time.
5. Tune sampling (temp, topK, topP) last. It changes output style more than runtime cost.

Model load tuning (ModelParams)#

const modelParams = ModelParams(
  contextSize: 4096,
  gpuLayers: ModelParams.maxGpuLayers,
  preferredBackend: GpuBackend.vulkan,
  numberOfThreads: 0,
  numberOfThreadsBatch: 0,
  batchSize: 0, // Native decoder default: min(contextSize, 2048).
  microBatchSize: 0, // Native decoder default: min(resolved batch, 512).
);

• preferredBackend: the biggest choice. Small models on mobile can be faster on cpu than on vulkan or WebGPU; larger models and longer responses favor the GPU. Always measure both.
• gpuLayers: start with the default and lower it if stability or latency is worse than on CPU.
• contextSize: keep it only as large as the use case needs. Oversized context raises first-token latency and memory use.
• numberOfThreads / numberOfThreadsBatch: 0 (automatic) is a good baseline. Some mobile devices prefer fewer threads; set them only after measuring.
• batchSize / microBatchSize: native decoder models start at the llama.cpp-aligned caps of 2048 and 512. Lower microBatchSize first (for example to 256 or 128) when memory or GPU stability is tight; bigger is not always faster. Encoder-only embedding models keep full-context native defaults for correctness; set both values explicitly for a known embedding workload (Embeddings).
• maxParallelSequences: matters for batched embeddings and true multi-sequence workloads, not single-turn chat.

WebGPU keeps full-context automatic batching because the bridge cannot report model architecture before context creation. Two presets apply when batchSize is 0:

• If the model URL contains gemma-4 or modelBytesHint is at least 2 GiB, the batch is capped at min(contextSize, 512). An explicit microBatchSize is kept up to that cap.
• If the model URL contains qwen3.5-0.8b, both batch sizes are unset, the backend is not cpu and gpuLayers is not 0, the sizes are 32 / 8.

URL matching ignores case.

For the existing Qwen3.5-0.8B URL preset, CPU loads (preferredBackend: GpuBackend.cpu or gpuLayers: 0) resolve unset sizes as native decoders do: a batch of min(contextSize, 2048) and a micro-batch of at most 512. This reduces temporary memory without reducing contextSize. Other models keep full-context automatic batching on CPU too: applying a 512-token micro-batch to a non-causal embedding model can abort on longer inputs, even with memory64. A renamed Qwen model URL does not select the preset; set batch sizes explicitly in that case.

Decoder-focused web apps can set 2048 / 512 explicitly after validating their model and browser.

Field reference: Runtime parameters. Native LiteRT-LM .litertlm loads have their own opt-in fields, listed in LiteRT-LM runtime controls. After changing the activation type or prefill chunk size, benchmark load time, prefill and decode throughput, and output quality on the deployment device.

Generation tuning (GenerationParams)#

const generationParams = GenerationParams(
  maxTokens: 256,
  temp: 0.7,
  topK: 40,
  topP: 0.9,
  minP: 0.0,
  penalty: 1.1,
  presencePenalty: 0.0,
  reusePromptPrefix: true,
  streamBatchTokenThreshold: 8,
  streamBatchByteThreshold: 512,
);

• maxTokens is a performance knob as much as a quality knob. Cap it aggressively on latency-sensitive paths.
• temp, topK, topP, penalty and presencePenalty shape output; they rarely fix a slow backend. Change them gradually, one at a time.
• penalty is a repetition penalty. presencePenalty is a separate llama.cpp-native control that penalizes any token already present in the recent window; one does not substitute for the other. WebGPU applies presencePenalty and minP only with bridge assets whose getCompletionCapabilities() reports them, and otherwise rejects a non-zero value, as LiteRT-LM does.
• streamBatchTokenThreshold / streamBatchByteThreshold (native): lower values give finer token-by-token UI updates; higher values raise throughput by reducing isolate message overhead.
• reusePromptPrefix is on by default for native generation. Keep it on for multi-turn chat and repeated prompts. Reuse targets evolving prompts with a shared prefix; an exact prompt replay is re-ingested to keep output deterministic. Validate parity for your model with the prompt-reuse parity tool (Reproducing).

Speculative decoding#

Speculative decoding is off by default and is not a universal speedup. Benchmark it on the target model and device, and compare deterministic output, acceptance and warmed throughput against the same baseline before enabling it in production.

• Native LiteRT-LM: set GenerationParams(speculativeDecoding: true). It was slower for Gemma 4 E2B on both measured devices (Backend benchmarks).
• Native llama.cpp: pass speculativeDecodingConfig. The legacy speculativeDecoding: true flag without a config runs ngram-mod.
• WebGPU: the same configs, with bridge assets whose getCompletionCapabilities() reports the strategy: bridge assets v0.1.54+, the default pin among them. draftModelPath and the n-gram cache paths are URLs, and mtp uses only the model's own MTP layers. See WebGPU bridge.
• LiteRT-LM web rejects speculative decoding.
• On llama.cpp, native or WebGPU, speculative decoding is text-only and cannot be combined with thinkingBudget or grammar.

engine.backendGenerationCapabilities reports the strategies the loaded runtime runs in speculativeDecodingStrategies.

SpeculativeDecodingConfig constructors mirror upstream llama.cpp --spec-type values:

const generationParams = GenerationParams(
  maxTokens: 256,
  temp: 0,
  speculativeDecodingConfig: SpeculativeDecodingConfig.ngramMapK(
    ngramSizeN: 4,
    ngramSizeM: 8,
  ),
);

Knobs:

• Draft-model strategies and ngram-cache: draftTokenMax caps the draft length per step.
• ngram-simple, ngram-map-k, ngram-map-k4v: ngramSizeM is the effective draft length, matching upstream's draft m-gram window; draftTokenMax does not cap them.
• ngram-mod: ngramTokenMax when set, otherwise draftTokenMax, otherwise the llama.cpp default.
• loadMtp (ModelParams): keep it false unless bundled MTP tensors will be used, because loading them costs memory. An external MTP draft model loads as MTP automatically.
• speculativeRollbackTokenMax (ModelParams): set it to at least the MTP draft token max for architectures that need rollback snapshots, such as Qwen3.5 MTP.

Draftless n-gram strategies depend on the workload. On prompts with little repetition they can produce no drafts and run slower than baseline; measured results are in Backend benchmarks.

DSpark#

DSpark (SpeculativeDecodingConfig.draftDspark(draftModelPath: ...)) is an experimental, opt-in llama.cpp external-draft strategy mapped to upstream draft-dspark. The default v0.5.0 runtime supports it, including speculators-format checkpoints and LFM2 target/draft pairs. It is never selected automatically, and support still depends on the target, draft and backend. If speculative initialization fails, the LlamaUnsupportedException names the minimum native tag, b10356.

DFlash draft models#

DFlash drafts must use upstream-compatible GGUF metadata: general.architecture=dflash plus the dflash.* metadata block, including dflash.target_layers. A known-good public pair is target unsloth/Qwen3.5-4B-GGUF (Qwen3.5-4B-Q4_K_M.gguf) with draft EntityDeletr/Qwen3.5-4B-DFlash-GGUF (Qwen3.5-4B-DFlash.gguf).

If the draft fails to load and native logs report unknown model architecture: 'dflash-draft' or missing DFlash target-layer metadata, the artifact uses general.architecture=dflash-draft or lacks dflash.target_layers. Reconvert or replace the draft GGUF; llamadart does not patch draft metadata at runtime.

Multimodal tuning#

• Reduce image size before anything else.
• Keep contextSize and maxTokens tighter than text-only defaults.
• If GPU multimodal is unstable, get a correct CPU baseline first, then revisit GPU and offload settings.
• Treat projector loading and multimodal generation as separate stages; one can be healthy while the other is slow or unstable.

Read the diagnostics#

• first: first-token latency. If high, look at model load, prompt size, context and prompt evaluation.
• total: end-to-end wall time.
• avg: throughput across the whole request.
• decode: steady-state generation speed once output starts.

Native llama.cpp timing fields:

• p_eval: prompt evaluation time. High values point to prompt and context overhead, not the sampler.
• eval: decode time for generated tokens. High values point to backend kernel or scheduler cost.
• sample: token selection overhead. Usually small; if large, check for unusual sampling settings.
• reuse: prompt-prefix reuse count. If it stays low in multi-turn chat, prefix reuse is not helping.

Check the active backend and VRAM where available:

final backendName = await engine.getBackendName();
final vram = await engine.getVramInfo();
print('$backendName total=${vram.total} free=${vram.free}');

Heuristics by environment#

• Mobile native: test CPU against GPU early; small models often favor CPU.
• Desktop native: the GPU pays off more as model size or response length grows.
• Browser: start with conservative GPU settings; browser GPU paths have tighter stability limits than native.
• Multimodal: expect stricter limits than text-only, especially on mobile and browser targets.

Compare fairly#

• Use the same model, prompt, contextSize, maxTokens and backend-specific limits across runs.
• Record latency and throughput; a setting that improves one can hurt the other.
• Validate memory behavior with your real context sizes.

Benchmark and parity scripts for a repository checkout, and measured llama.cpp and LiteRT-LM results, are in Backend benchmarks.