[doc] explain the class hierarchy in vLLM (vllm-project#10240)

Signed-off-by: youkaichao <[email protected]>

docs/source/design/class_hierarchy.rst

@@ -0,0 +1,33 @@
+vLLM's Class Hierarchy
+=======================
+
+This document describes the class hierarchy of vLLM. We will explain the relationships between the core classes, their responsibilities, and the design choices behind them to make vLLM more modular and extensible.
+
+1. **Entrypoints**: vLLM has two entrypoints: `command line usage <https://github.com/vllm-project/vllm/blob/d1c6799b8870e513bf4f2305cbf6cda9fc3d773b/vllm/entrypoints/api_server.py#L138>`__ with ``vllm serve`` for launching an OpenAI-API compatible server, and `library-style usage <https://github.com/vllm-project/vllm/blob/d1c6799b8870e513bf4f2305cbf6cda9fc3d773b/vllm/entrypoints/llm.py#L38>`__ with the ``vllm.LLM`` class for running inference in a Python script. These are user-facing entrypoints that end-users interact with. Under the hood, both create an engine object to handle model inference.
+
+2. **Engine**: Each vLLM instance contains one engine object, orchestrating and serving as the control plane for model inference. Depending on the configuration, the engine can create multiple workers to handle the inference workload.
+
+3. **Worker**: A worker is a process that runs the model inference. vLLM follows the common practice of using one process to control one accelerator device, such as GPUs. For example, if we use tensor parallelism of size 2 and pipeline parallelism of size 2, we will have 4 workers in total. Workers are identified by their ``rank`` and ``local_rank``. ``rank`` is used for global orchestration, while ``local_rank`` is mainly used for assigning the accelerator device and accessing local resources such as the file system and shared memory.
+
+4. **Model Runner**: Every worker has one model runner object, responsible for loading and running the model. Much of the model execution logic resides here, such as preparing input tensors and capturing cudagraphs.
+
+5. **Model**: Every model runner object has one model object, which is the actual ``torch.nn.Module`` instance. See :ref:`huggingface_integration` for how various configurations affect the class we ultimately get.
+
+The following figure shows the class hierarchy of vLLM:
+
+    .. figure:: ../assets/design/hierarchy.png
+        :alt: query
+        :width: 100%
+        :align: center
+
+There are several important design choices behind this class hierarchy:
+
+1. **Extensibility**: All classes in the hierarchy accept a configuration object containing all the necessary information. The `VllmConfig <https://github.com/vllm-project/vllm/blob/d1c6799b8870e513bf4f2305cbf6cda9fc3d773b/vllm/config.py#L2036>`__ class is the main configuration object that is passed around. The class hierarchy is quite deep, and every class needs to read the configuration it is interested in. By encapsulating all configurations in one object, we can easily pass the configuration object around and access the configuration we need. Suppose we want to add a new feature (this is often the case given how fast the field of LLM inference is evolving) that only touches the model runner. We will have to add a new configuration option in the `VllmConfig` class. Since we pass the whole config object around, we only need to add the configuration option to the `VllmConfig` class, and the model runner can access it directly. We don't need to change the constructor of the engine, worker, or model class to pass the new configuration option.
+
+2. **Uniformity**: The model runner needs a unified interface to create and initialize the model. vLLM supports more than 50 types of popular open-source models. Each model has its own initialization logic. If the constructor signature varies with models, the model runner does not know how to call the constructor accordingly, without complicated and error-prone inspection logic. By making the constructor of the model class uniform, the model runner can easily create and initialize the model without knowing the specific model type. This is also useful for composing models. Vision-language models often consist of a vision model and a language model. By making the constructor uniform, we can easily create a vision model and a language model and compose them into a vision-language model.
+
+3. **Sharding and Quantization at Initialization**: Certain features require changing the model weights. For example, tensor parallelism needs to shard the model weights, and quantization needs to quantize the model weights. There are two possible ways to implement this feature. One way is to change the model weights after the model is initialized. The other way is to change the model weights during the model initialization. vLLM chooses the latter. The first approach is not scalable to large models. Suppose we want to run a 405B model (with roughly 810GB weights) with 16 H100 80GB GPUs. Ideally, every GPU should only load 50GB weights. If we change the model weights after the model is initialized, we need to load the full 810GB weights to every GPU and then shard the weights, leading to a huge memory overhead. Instead, if we shard the weights during the model initialization, every layer will only create a shard of the weights it needs, leading to a much smaller memory overhead. The same idea applies to quantization. Note that we also add an additional argument ``prefix`` to the model's constructor so that the model can initialize itself differently based on the prefix. This is useful for non-uniform quantization, where different parts of the model are quantized differently. The ``prefix`` is usually an empty string for the top-level model and a string like ``"vision"`` or ``"language"`` for the sub-models. In general, it matches the name of the module's state dict in the checkpoint file.
+
+One disadvantage of this design is that it is hard to write unit tests for individual components in vLLM because every component needs to be initialized by a complete config object. We solve this problem by providing a default initialization function that creates a default config object with all fields set to ``None``. If the component we want to test only cares about a few fields in the config object, we can create a default config object and set the fields we care about. This way, we can test the component in isolation. Note that many tests in vLLM are end-to-end tests that test the whole system, so this is not a big problem.
+
+In summary, the complete config object ``VllmConfig`` can be treated as an engine-level global state that is shared among all vLLM classes.

docs/source/design/huggingface_integration.rst

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+.. _huggingface_integration:
+
 Integration with HuggingFace
 ===================================
 

docs/source/index.rst

@@ -156,10 +156,11 @@ Documentation
    :maxdepth: 2
    :caption: Design
 
+   design/class_hierarchy
+   design/huggingface_integration
    design/input_processing/model_inputs_index
    design/kernel/paged_attention
    design/multimodal/multimodal_index
-   design/huggingface_integration
 
 .. For Developers: contributing to the vLLM project