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ModelComputeOptions configures which compute units (CPU, GPU, Neural Engine) are used for each component of the WhisperKit transcription pipeline.

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Overview

WhisperKit models consist of multiple components that can run on different compute units. ModelComputeOptions allows fine-grained control over where each component executes for optimal performance.

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Properties

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melCompute
MLComputeUnits
default:".cpuAndGPU"
Compute units for mel spectrogram generation.
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audioEncoderCompute
MLComputeUnits
Compute units for audio encoding. Defaults to Neural Engine on newer OS versions for better performance.
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textDecoderCompute
MLComputeUnits
default:".cpuAndNeuralEngine"
Compute units for text decoding.
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prefillCompute
MLComputeUnits
default:".cpuOnly"
Compute units for KV cache prefill operations.

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Initialization

public init(
    melCompute: MLComputeUnits = .cpuAndGPU,
    audioEncoderCompute: MLComputeUnits? = nil,
    textDecoderCompute: MLComputeUnits = .cpuAndNeuralEngine,
    prefillCompute: MLComputeUnits = .cpuOnly
)

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Parameters

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melCompute
MLComputeUnits
default:".cpuAndGPU"
Compute units for mel spectrogram extraction.
​
audioEncoderCompute
MLComputeUnits
default:"nil"
Compute units for audio encoding. When nil, automatically selects .cpuAndNeuralEngine on iOS 17+/macOS 14+ and .cpuAndGPU on older versions.
​
textDecoderCompute
MLComputeUnits
default:".cpuAndNeuralEngine"
Compute units for text decoding.
​
prefillCompute
MLComputeUnits
default:".cpuOnly"
Compute units for prefill operations.

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MLComputeUnits Values

Apple’s MLComputeUnits enum specifies which processors can execute a Core ML model:
  • .cpuOnly - Only the CPU
  • .cpuAndGPU - CPU and GPU
  • .cpuAndNeuralEngine - CPU and Neural Engine (recommended for most models)
  • .all - All available compute units

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Usage Examples

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Default Configuration

let options = ModelComputeOptions()
// Uses optimal defaults for your device

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Custom Configuration

let options = ModelComputeOptions(
    melCompute: .cpuAndGPU,
    audioEncoderCompute: .cpuAndNeuralEngine,
    textDecoderCompute: .cpuAndNeuralEngine,
    prefillCompute: .cpuOnly
)

let config = WhisperKitConfig(
    model: "openai_whisper-base",
    computeOptions: options
)

let whisperKit = try await WhisperKit(config)

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Force CPU-Only Execution

let cpuOnlyOptions = ModelComputeOptions(
    melCompute: .cpuOnly,
    audioEncoderCompute: .cpuOnly,
    textDecoderCompute: .cpuOnly,
    prefillCompute: .cpuOnly
)

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Simulator Handling

When running on the iOS Simulator, all compute options are automatically overridden to .cpuOnly since the simulator doesn’t support GPU or Neural Engine execution.

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Performance Considerations

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Neural Engine

The Neural Engine provides the best performance for most Whisper models on Apple Silicon devices:
  • Recommended for audioEncoderCompute and textDecoderCompute
  • Requires iOS 17+/macOS 14+ for optimal audio encoder performance
  • Most efficient for battery-powered devices

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GPU

The GPU can be beneficial for certain operations:
  • Recommended for melCompute on most devices
  • May provide better throughput on some Mac models
  • Consider for batch processing scenarios

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CPU Only

CPU-only execution:
  • Required for prefillCompute operations
  • Use when Neural Engine/GPU are unavailable
  • May be necessary for debugging or compatibility

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