Architecture Ovewiew

TensorRT LLM is a toolkit designed to create optimized solutions for Large Language Model (LLM) inference. Besides TensorRT, PyTorch can also serve as the backend for TensorRT-LLM. This document provides an overview of the PyTorch Backend architecture.

Top Level API

The interface for PyTorch backend is tensorrt_llm.LLM.

from tensorrt_llm import LLM
llm = LLM(model=<path_to_llama_from_hf>)

The LLM also manages the tokenization and detokenization processes of the input.

PyExecutor

Similar to the TensorRT backend, which uses Executor API, the PyTorch backend employs a PyExecutor class. This class has a similar interface to Executor, allowing it to be integrated into LLM as an alternative backend. Key components of the PyExecutor include:

Model Engine: Holds the language model and efficiently supports single-step model forward.
Decoder: Generates output tokens based on Model Engine outputs. Currently, only greedy search is supported.
Scheduler: Decides whether to allocate resources (like KV Cache) for a request and whether to run forward for each request at the current step.

The single-step flow of PyExecutor involves:

Fetching new requests from the request queue, if any.
Scheduling some requests.
Running model forward for scheduled requests.
Running the decoder using the model forward outputs for the scheduled requests.
Adding output tokens for each request and handling finished requests.

Model Engine

The core component of PyExecutor is the ModelEngine, responsible for executing the model's forward pass efficiently on the GPU. The key method of ModelEngine is forward, which handles the forward pass computation. For the PyTorch backend, the derived class is PyTorchModelEngine, declared in model_engine.py.

Decoder

The Decoder generates output tokens based on Model Engine outputs and supports greedy search decoding.

Scheduler

The scheduler operates in two steps:

CapacityScheduler: Determines if there are enough resources to accommodate a request.
MicroBatchScheduler: Selects some requests for the model to run forward.

Both CapacityScheduler and MicroBatchScheduler currently use C++ bindings. However, since the interfaces are implemented in Python, customization is possible. The document scheduler.md explains how to implement customized scheduling logic.

ResourceManager

ResourceManager helps allocate and manage these resources that may be needed to run inference for a single request. It is a container of objects inherited from BaseResourceManager, each managing a specific type of resource. There are three important interfaces for BaseResourceManager:

prepare_resources: Called at each step before model forward in PyExecutor for the current batch.
update_resources: Called at each step finish for the current batch.
free_resources: Called at each request finish.

One crucial resource is the KV Cache for transformer models. The BaseResourceManager for KV Cache is KVCacheManager.

KVCacheManager

Currently, the KVCacheManager uses C++ binding. However, customization in Python is possible, as its interface is implemented in Python. The document kv_cache_manager.md details how to implement a customized KVCacheManager.

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