PEFT vs vLLM

Injects trainable low-rank decomposition matrices (LoRA) into transformer attention and feed-forward layers by wrapping linear modules with a registry-based dispatch system. Uses PeftModel wrapper pattern to intercept forward passes and compose base weights with adapter weights via matrix multiplication, enabling training of only 0.1-2% of parameters while maintaining architectural compatibility with HuggingFace transformers.

Unique: Uses a registry-based tuner dispatch system (src/peft/mapping.py) that maps PEFT method names to concrete tuner classes, enabling dynamic adapter injection without modifying base model code. The PeftModel wrapper (src/peft/peft_model.py 72-1478) intercepts forward passes and composes adapter outputs with base model outputs, maintaining full compatibility with HuggingFace's model hub and distributed training frameworks.

vs alternatives: Achieves 10-100x smaller checkpoints than full fine-tuning while maintaining performance comparable to full-parameter training, with native integration into the HuggingFace ecosystem (no custom model definitions required)

Extends LoRA with automatic rank discovery by computing importance scores for adapter parameters during training and pruning low-importance weights. Implements a parametric allocation algorithm that adjusts per-layer ranks dynamically based on gradient statistics, reducing manual hyperparameter tuning while maintaining task performance with fewer total parameters than fixed-rank LoRA.

Unique: Implements parametric rank allocation (src/peft/tuners/adalora.py) that computes importance scores from gradient statistics and applies structured pruning to adapter matrices during training. Unlike static LoRA, AdaLoRA adjusts per-layer ranks based on task-specific importance, automatically discovering which layers need higher capacity.

vs alternatives: Achieves better parameter efficiency than fixed-rank LoRA by discovering layer-specific optimal ranks automatically, eliminating manual rank search while maintaining or improving downstream task performance

Uses a declarative configuration system (PeftConfig subclasses) that specifies adapter type, hyperparameters, and target modules, enabling adapter creation without writing custom code. Implements a registry-based factory pattern (src/peft/mapping.py) that maps configuration objects to concrete tuner implementations, supporting 25+ PEFT methods through unified configuration interface.

Unique: Implements a registry-based configuration system (src/peft/config.py and src/peft/mapping.py) where each PEFT method has a dedicated PeftConfig subclass that specifies hyperparameters and target modules. The factory pattern maps configurations to concrete tuner implementations, enabling 25+ methods through a unified interface.

vs alternatives: Enables rapid experimentation across 25+ PEFT methods through declarative configuration, eliminating need for custom code per method while maintaining reproducibility via JSON serialization

Allows fine-grained control over which model layers receive adapters through pattern matching on module names (e.g., 'q_proj', 'v_proj' for attention, 'mlp' for feed-forward). Implements regex-based and exact-match module selection that enables adapting only specific layers (e.g., attention layers only) without modifying feed-forward layers, reducing parameters and enabling layer-specific optimization.

Unique: Implements flexible module selection via target_modules parameter that supports exact matching and regex patterns (src/peft/peft_model.py), enabling adapters to be applied to specific layers without modifying others. Supports layer-wise customization of adapter hyperparameters through per-module configuration.

vs alternatives: Enables fine-grained control over adapter placement, allowing practitioners to optimize parameter count and performance by adapting only specific layers (e.g., attention only) rather than all layers

Integrates with PyTorch's gradient checkpointing to trade computation for memory by recomputing activations during backward pass instead of storing them. Automatically enables gradient checkpointing for adapter training, reducing peak memory usage by 30-50% while adding ~20-30% training time overhead, enabling larger batch sizes on memory-constrained hardware.

Unique: Integrates PyTorch's gradient checkpointing mechanism with adapter training to enable memory-efficient fine-tuning by recomputing activations during backward pass. Works transparently with PEFT adapters, reducing peak memory by 30-50% with minimal code changes.

vs alternatives: Reduces peak memory usage by 30-50% during adapter training by trading computation for memory, enabling larger batch sizes and training on more memory-constrained hardware

Enables training adapters in mixed precision (float16 or bfloat16) with automatic loss scaling to prevent gradient underflow, reducing memory usage by 50% and improving training speed by 1.5-2x. Integrates with PyTorch's automatic mixed precision (AMP) and transformers' native mixed-precision support to maintain numerical stability while reducing precision.

Unique: Integrates PyTorch's automatic mixed precision (AMP) with PEFT adapter training, enabling float16/bfloat16 computation while maintaining numerical stability through automatic loss scaling. Works transparently with all PEFT methods and distributed training frameworks.

vs alternatives: Reduces memory usage by 50% and improves training speed by 1.5-2x using mixed precision, with minimal performance degradation (1-2%) compared to full-precision training

Enables selecting and routing to different adapters at inference time based on input characteristics or external signals, without reloading base model weights. Implements set_adapter() method that switches active adapter in-place, enabling dynamic adapter selection in production systems where different inputs may require different task-specific adapters.

Unique: Implements in-place adapter switching via set_adapter() method (src/peft/peft_model.py) that changes active adapter without reloading base model, enabling dynamic routing at inference time. Supports composition of multiple adapters for ensemble effects.

vs alternatives: Enables dynamic adapter selection at inference time without reloading base model, supporting multi-task and multi-tenant inference scenarios with minimal latency overhead

Prepends learnable prefix tokens to input embeddings that are optimized during fine-tuning, allowing the model to learn task-specific prompts without modifying base model weights. Implements a shallow feed-forward network that projects prefix parameters to full embedding dimension, enabling efficient adaptation by training only prefix embeddings (typically 0.1-1% of model size).

Unique: Implements prefix tuning via a learnable embedding matrix that is prepended to input sequences, with optional projection through a shallow feed-forward network (src/peft/tuners/prefix_tuning.py). Unlike LoRA which modifies internal weights, prefix tuning learns task-specific prompts that guide the frozen base model, enabling true prompt-based adaptation.

vs alternatives: Enables prompt-based adaptation without modifying model weights, making it ideal for scenarios where prompt engineering is preferred or where multiple task-specific prefixes must coexist on the same base model

Implements virtual memory-inspired paging for KV cache blocks, allowing non-contiguous memory allocation and reuse across requests. Prefix caching enables sharing of computed attention keys/values across requests with common prompt prefixes, reducing redundant computation. The KV cache is managed through a block allocator that tracks free/allocated blocks and supports dynamic reallocation during generation, achieving 10-24x throughput improvement over dense allocation schemes.

Unique: Uses block-level virtual memory abstraction for KV cache instead of contiguous allocation, combined with prefix caching that detects and reuses computed attention states across requests with identical prompt prefixes. This dual approach (paging + prefix sharing) is not standard in other inference engines like TensorRT-LLM or vLLM competitors.

vs alternatives: Achieves 10-24x higher throughput than HuggingFace Transformers by eliminating KV cache fragmentation and recomputation through paging and prefix sharing, whereas alternatives typically allocate fixed contiguous buffers or lack prefix-level cache reuse.

Implements a scheduler that decouples request arrival from batch formation, allowing new requests to be added mid-generation and completed requests to be removed without waiting for batch boundaries. The scheduler maintains request state (InputBatch) tracking token counts, generation progress, and sampling parameters per request. Requests are dynamically scheduled based on available GPU memory and compute capacity, enabling variable batch sizes that adapt to request completion patterns rather than fixed-size batches.

Unique: Decouples request arrival from batch formation using an event-driven scheduler that tracks per-request state (InputBatch) and dynamically adjusts batch composition mid-generation. Unlike static batching, requests can be added/removed at any generation step, and the scheduler adapts batch size based on GPU memory availability rather than fixed batch size configuration.

vs alternatives: Achieves higher throughput than static batching (used in TensorRT-LLM) by eliminating idle time when requests complete at different rates, and lower latency than fixed-batch systems by immediately scheduling short requests rather than waiting for batch boundaries.