一、前言
随着 LLM 模型越来越大，单 GPU 已经无法加载一个模型。以 Qwen-14B-Chat 模型为例，模型权重大概 28GB，但是单个 NVIDIA A10 仅有 24GB 显存。如果想要在 A10 上部署 Qwen-14B-Chat 模型，我们需要将模型切分后部署到 2 个 A10 机器上，每个 A10 卡加载一半的模型，这种方式称之为分布式推理。
社区涌现了很多支持分布式推理的框架如 vllm、deepspeed-mii，rtp-llm 等。本文选取了 vllm 框架，从源码角度分析 vllm + Ray 如何实现 LLM 模型的分布式推理。
二、在 K8s 中部署 vllm 分布式推理应用
2.1 模型准备
下载 Qwen-14B-Chat 到 OSS 中，并在集群中创建对应的 pv，pvc。pvc 名称为 llm-model。
kubectl apply -f- << EOFapiVersion: v1kind: Secretmetadata: name: oss-secretstringData: akId: ${your-accesskey-id} # 用于访问oss的AK akSecret: ${your-accesskey-secert} # 用于访问oss的SK---apiVersion: v1kind: PersistentVolumemetadata: name: llm-model labels: alicloud-pvname: llm-modelspec: capacity: storage: 30Gi accessModes: - ReadOnlyMany persistentVolumeReclaimPolicy: Retain csi: driver: ossplugin.csi.alibabacloud.com volumeHandle: model-oss nodePublishSecretRef: name: oss-secret namespace: default volumeAttributes: bucket: ${your-bucket-name} url: ${your-bucket-endpoint} # e.g. oss-cn-hangzhou.aliyuncs.com otherOpts: "-o umask=022 -o max_stat_cache_size=0 -o allow_other" path: "/"---apiVersion: v1kind: PersistentVolumeClaimmetadata: name: llm-modelspec: accessModes: - ReadOnlyMany resources: requests: storage: 30Gi selector: matchLabels: alicloud-pvname: llm-modelEOF
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2.2 部署分布式 vllm 应用
1. 执行以下命令，部署 vllm 应用
kubectl apply -f- <<EOFapiVersion: apps/v1 kind: Deploymentmetadata: name: vllm labels: app: vllmspec: replicas: 2 selector: matchLabels: app: vllm template: metadata: labels: app: vllm spec: affinity: podAntiAffinity: requiredDuringSchedulingIgnoredDuringExecution: - labelSelector: matchExpressions: - key: app operator: In values: - vllm topologyKey: kubernetes.io/hostname volumes: - name: model persistentVolumeClaim: claimName: llm-model containers: - name: vllm image: kube-ai-registry.cn-shanghai.cr.aliyuncs.com/kube-ai/vllm:0.4.1 command: - "sh" - "-c" - "sleep 7d" ports: - containerPort: 8080 readinessProbe: tcpSocket: port: 8080 initialDelaySeconds: 30 periodSeconds: 30 resources: limits: nvidia.com/gpu: "1" requests: cpu: 4 memory: 8Gi nvidia.com/gpu: "1" volumeMounts: - mountPath: /mnt/models name: modelEOF
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2. 执行以下命令，启动 vllm 应用
ray start --head# 启动后，日志中会显示ray-head-address地址
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# ray-head-address 设置为pod1日志中显示的ray-head-address地址ray start --address=<ray-head-address>
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python3 model_init.py
from transformers import AutoModelForCausalLM, AutoTokenizer, AutoConfig
config = AutoConfig.from_pretrained( "/mnt/models/Qwen-14B-Chat", trust_remote_code=True)tokenizer = AutoTokenizer.from_pretrained("/mnt/models/Qwen-14B-Chat", trust_remote_code=True)
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python3 -m vllm.entrypoints.openai.api_server \--port 8080 \--trust-remote-code \--served-model-name qwen \--model /mnt/models/Qwen-14B-Chat \--gpu-memory-utilization 0.95 \--tensor-parallel-size 2
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kubectl -n <your-namespace> exec -it <pod1-name> bash
curl -H "Content-Type: application/json" \ http://localhost:8080/v1/chat/completions -X POST \ -d '{"model": "qwen", "messages": [{"role": "user", "content": "你好"}], "max_tokens": 512, "temperature": 0.7, "top_p": 0.9, "seed": 10, "stop":["<|endoftext|>", "<|im_end|>", "<|im_start|>"]}'
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三、分布式推理总体流程分析
1.入口函数：vllm/entrypoints/openai/api_server.py main
if __name__ == "__main__": # 构建engine args engine_args = AsyncEngineArgs.from_cli_args(args) # 构建engine engine = AsyncLLMEngine.from_engine_args( engine_args, usage_context=UsageContext.OPENAI_API_SERVER)
openai_serving_chat = OpenAIServingChat(engine, served_model_names, args.response_role, args.lora_modules, args.chat_template)
openai_serving_completion = OpenAIServingCompletion( engine, served_model_names, args.lora_modules)
app.root_path = args.root_path uvicorn.run(app）
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2.构建 LLM engine
engine = AsyncLLMEngine.from_engine_args( engine_args, usage_context=UsageContext.OPENAI_API_SERVER)
def from_engine_args(): """Creates an async LLM engine from the engine arguments.""" # Create the engine configs. engine_config = engine_args.create_engine_config()
# ray 集群初始化 initialize_ray_cluster(engine_config.parallel_config) from vllm.executor.ray_gpu_executor import RayGPUExecutorAsync executor_class = RayGPUExecutorAsync
# Create the engine configs. engine_config = engine_args.create_engine_config()
# ray 集群初始化 # 1. ray.init() # 2. 根据集群内gpu数量 & tp并发度设置ray placement策略 initialize_ray_cluster(engine_config.parallel_config) from vllm.executor.ray_gpu_executor import RayGPUExecutorAsync executor_class = RayGPUExecutorAsync
# Create the async LLM engine. engine = cls(...) #创建一个AsyncLLMEngine实例 # AsyncLLMEngine.__init__ -> self._init_engine -> _AsyncLLMEngine.__init__ -> LLMEngine.__init__ -> executor_class() 即调用RayGPUExecutorAsync.__init__
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3.初始化 Ray 集群
Ray Worker 初始化包括 Ray 集群初始化，Ray Worker 初始化。在 Ray worker 初始化时会分布式加载模型。
# RayGPUExecutorAsync 继承了RayGPUExecutor及ExecutorAsyncBase 类，初始化时会调用RayGPUExecutor的self._init_executor 方法def _init_executor(self) -> None: # Create the parallel GPU workers. 初始化workers 核心代码 self._init_workers_ray(placement_group)
def _init_workers_ray(): # 定义worker, 是vllm.worker.worker模块里的Worker类 # actor为RayWorkerWrapper类 worker = ray.remote( num_cpus=0, num_gpus=num_gpus, scheduling_strategy=scheduling_strategy, **ray_remote_kwargs, )(RayWorkerWrapper).remote( worker_module_name="vllm.worker.worker", worker_class_name="Worker", trust_remote_code=self.model_config.trust_remote_code, )
# 在Ray Worker上依次执行如下方法 self._run_workers("get_node_and_gpu_ids", use_dummy_driver=True) self._run_workers("update_environment_variables", all_args=all_args_to_update_environment_variables) self._run_workers("init_worker", all_kwargs=init_worker_all_kwargs) self._run_workers("init_device") self._run_workers( "load_model", max_concurrent_workers=self.parallel_config. max_parallel_loading_workers, )
def _run_workers(): # Start the ray workers first. ray_worker_outputs = [ # worker是前面定义的RayWorkerWrapper类, 继承RayWorkerWrapper类 # 实际调用了RayWorkerWrapper.execute_method 并在远程实例上执行method方法 worker.execute_method.remote(method, *worker_args, **worker_kwargs) for (worker, worker_args, worker_kwargs ) in zip(self.workers, all_worker_args, all_worker_kwargs) ]
def init_worker(): # worker_module_name 是 vllm.worker.worker 就是_init_workers_ray方法中传入的 mod = importlib.import_module(self.worker_module_name) # Worker worker_class = getattr(mod, self.worker_class_name) self.worker = worker_class(*args, **kwargs) # Worker.__init__ -> ModelRunner.__init__
def init_device(): # 初始化分布式推理的机器信息 """Initialize the distributed environment.""" init_distributed_environment(parallel_config.world_size, rank, distributed_init_method, local_rank)
def load_model(): self.model_runner.load_model() # ModelRunner.load_model() -> vllm.model_executor.model_loader.loader.load_model
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执行完 load_model()的预期日志输出如下，可以看到两个 pod，每个加载了 13.2845 GB，即一半的模型。
INFO 04-26 09:39:46 model_runner.py:173] Loading model weights took 13.2845 GB(RayWorkerWrapper pid=3327, ip=192.168.12.132) INFO 04-26 09:39:51 model_runner.py:173] Loading model weights took 13.2845 GB
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4.对外提供服务
创建 OpenAIServingChat 以及 OpenAIServingCompletion 实例，启动 uvicorn 对外提供服务。
@app.post("/v1/chat/completions")openai_serving_chat = OpenAIServingChat(engine, served_model_names, args.response_role, args.lora_modules, args.chat_template)@app.post("/v1/completions")openai_serving_completion = OpenAIServingCompletion( engine, served_model_names, args.lora_modules)
app.root_path = args.root_pathuvicorn.run(app）
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3.1 分布式推理过程
当启动参数--tensor-parallel-size > 1 时，会自动触发 ray 分布式部署。
1. 构建 LLM engine 时会对 Ray 集群进行初始化
# ray 集群初始化initialize_ray_cluster(engine_config.parallel_config)
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parallel_config 的配置如下，pp=1，tp=2，world_size=2
{'pipeline_parallel_size': 1, 'tensor_parallel_size': 2, 'worker_use_ray': True, 'max_parallel_loading_workers': None, 'disable_custom_all_reduce': False, 'tokenizer_pool_config': None, 'ray_workers_use_nsight': False, 'placement_group': None, 'world_size': 2}
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初始化时会为 worker 进程创建 placement_group。
1）获取 ray cluster 中所有 gpu 的数量。
2）根据 world size 申请 gpu placement_group_specs = ([{"GPU": 1}] * parallel_config.world_size)。
3）创建 placement_group，ray 会根据 placement_group 在对应 node 上启动 actor。
2. 在每个 worker 上执行 get_node_and_gpu_ids 方法
# 获取node及node上分配的gpu卡信息def get_node_and_gpu_ids(self) -> Tuple[str, List[int]]: node_id = ray.get_runtime_context().get_node_id() gpu_ids = ray.get_gpu_ids() return node_id, gpu_ids
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3. 在每个 worker 上执行 update_environment_variables
# 第二步获取的worker_node以及gpu信息worker_node_and_gpu_ids = self._run_workers("get_node_and_gpu_ids", use_dummy_driver=True)
# Set environment variables for the driver and workers.all_args_to_update_environment_variables = [({ "CUDA_VISIBLE_DEVICES": ",".join(map(str, node_gpus[node_id])), "VLLM_INSTANCE_ID": VLLM_INSTANCE_ID, "VLLM_TRACE_FUNCTION": os.getenv("VLLM_TRACE_FUNCTION", "0"), }, ) for (node_id, _) in worker_node_and_gpu_ids]
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4. 在每个 worker 上执行 init_device 方法
# worker的启动参数init_worker_all_kwargs = []# worker_node_and_gpu_ids 是第二步获取的worker上的gpu信息for rank, (node_id, _) in enumerate(worker_node_and_gpu_ids): local_rank = node_workers[node_id].index(rank) init_worker_all_kwargs.append( collect_arg_helper_func( model_config=self.model_config, parallel_config=self.parallel_config, scheduler_config=self.scheduler_config, device_config=self.device_config, cache_config=self.cache_config, load_config=self.load_config, local_rank=local_rank, rank=rank, distributed_init_method=distributed_init_method, lora_config=self.lora_config, vision_language_config=self.vision_language_config, is_driver_worker=rank == 0, ))
def init_device(self) -> None: if self.device_config.device.type == "cuda": # torch.distributed.all_reduce does not free the input tensor until # the synchronization point. This causes the memory usage to grow # as the number of all_reduce calls increases. This env var disables # this behavior. # Related issue: # https://discuss.pytorch.org/t/cuda-allocation-lifetime-for-inputs-to-distributed-all-reduce/191573 os.environ["TORCH_NCCL_AVOID_RECORD_STREAMS"] = "1"
# This env var set by Ray causes exceptions with graph building. os.environ.pop("NCCL_ASYNC_ERROR_HANDLING", None) self.device = torch.device(f"cuda:{self.local_rank}") torch.cuda.set_device(self.device)
_check_if_gpu_supports_dtype(self.model_config.dtype) torch.cuda.empty_cache() self.init_gpu_memory = torch.cuda.mem_get_info()[0] else: raise RuntimeError( f"Not support device type: {self.device_config.device}") # Initialize the distributed environment. init_worker_distributed_environment(self.parallel_config, self.rank, self.distributed_init_method, self.local_rank) # Set random seed. set_random_seed(self.model_config.seed)
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核心方法 init_worker_distributed_environment 用于构建分布式集群的 world 信息，类似 horovod 及 deepspeed 框架中的 world info。
该方法参数如下：
work1: self.rank=0, self.local_rank=0, self.distributed_init_method="tcp://192.168.12.120:42167" (ray master)
{'pipeline_parallel_size': 1, 'tensor_parallel_size': 2, 'worker_use_ray': True, 'max_parallel_loading_workers': None, 'disable_custom_all_reduce': False, 'tokenizer_pool_config': None, 'ray_workers_use_nsight': False, 'placement_group': <ray.util.placement_group.PlacementGroup object at 0x7fdeaa896ce0>, 'world_size': 2}, {'id': PlacementGroupID(51489eb26a9335f31ed1bdb4eace04000000), 'bundle_cache': [{'GPU': 1.0}, {'GPU': 1.0}]}, self.rank=0, tcp://192.168.12.120:42167, self.local_rank=0
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work2: self.rank=1,
self.local_rank=0,self.distributed_init_method="tcp://192.168.12.120:42167"
{'pipeline_parallel_size': 1, 'tensor_parallel_size': 2, 'worker_use_ray': True, 'max_parallel_loading_workers': None, 'disable_custom_all_reduce': False, 'tokenizer_pool_config': None, 'ray_workers_use_nsight': False, 'world_size': 2}, self.rank=1, tcp://192.168.12.120:42167, self.local_rank=0
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self.rank 全局递增，self.local_rank 是指在一个 pod 内第几个 gpu。
5. 在每个 worker 执行 load_model 方法
load_model 用于加载分布式模型，比较复杂，在下面的章节中单独介绍。
3.2 分布式模型加载流程
在每个 worker 执行 load_model 方法
def load_model(): self.model_runner.load_model()
# ModelRunner.load_model() -> vllm.model_executor.model_loader.loader.load_modeldef load_model(self) -> None: with CudaMemoryProfiler() as m: # get_model 获取模型 self.model = get_model( model_config=self.model_config, device_config=self.device_config, load_config=self.load_config, lora_config=self.lora_config, vision_language_config=self.vision_language_config, parallel_config=self.parallel_config, scheduler_config=self.scheduler_config, )
self.model_memory_usage = m.consumed_memory logger.info(f"Loading model weights took " f"{self.model_memory_usage / float(2**30):.4f} GB")
# get_model -> loader.load_model -> DefaultModelLoader.load_modeldef load_model(self, *, model_config: ModelConfig, device_config: DeviceConfig, lora_config: Optional[LoRAConfig], vision_language_config: Optional[VisionLanguageConfig], parallel_config: ParallelConfig, scheduler_config: SchedulerConfig) -> nn.Module: with set_default_torch_dtype(model_config.dtype): with torch.device(device_config.device): """Initialize a model with the given configurations.""" # 初始化模型 model = _initialize_model(model_config, self.load_config, lora_config, vision_language_config)
# 调用对应model的load_weights方法 model.load_weights( self._get_weights_iterator(model_config.model, model_config.revision, fall_back_to_pt=getattr( model, "fall_back_to_pt_during_load", True)), ) for _, module in model.named_modules(): linear_method = getattr(module, "linear_method", None) if linear_method is not None: linear_method.process_weights_after_loading(module) if hasattr(module, "process_weights_after_loading"): module.process_weights_after_loading() return model.eval()
# 根据model config找到具体是什么模型def _initialize_model( model_config: ModelConfig, load_config: LoadConfig, lora_config: Optional[LoRAConfig], vision_language_config: Optional[VisionLanguageConfig]) -> nn.Module: """Initialize a model with the given configurations.""" # Qwen-7B-Chat/config.json中architecture字段 model_class = get_model_architecture(model_config)[0] linear_method = _get_linear_method(model_config, load_config)
return model_class(config=model_config.hf_config, linear_method=linear_method, **_get_model_initialization_kwargs( model_class, lora_config, vision_language_config))
# model_class 是 <class 'vllm.model_executor.models.qwen.QWenLMHeadModel'>
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model.load_weights 即调用 QwenLMHeadModel 的 load_weights 方法
# QWenLMHeadModel.load_weightsdef load_weights(self, weights: Iterable[Tuple[str, torch.Tensor]]): stacked_params_mapping = [ # (param_name, shard_name, shard_id) ("gate_up_proj", "w2", 0), ("gate_up_proj", "w1", 1), ]
# 模型每层权重及其名称 # self.named_parameters即model.named_parameters() params_dict = dict(self.named_parameters())
for name, loaded_weight in weights: # name: transformer.h.27.mlp.c_proj.weight # loaded_weight: tensor(xxx) if "rotary_emb.inv_freq" in name: continue for (param_name, weight_name, shard_id) in stacked_params_mapping: if weight_name not in name: continue # 如果在stacked_params_mapping里，就需要把shard_name改为param_name # 如 name为 transformer.h.0.mlp.w1.weight，则name需要改为 transformer.h.0.mlp.gate_up_proj.weight name = name.replace(weight_name, param_name) # Skip loading extra bias for GPTQ models. if name.endswith(".bias") and name not in params_dict: continue param = params_dict[name] weight_loader = param.weight_loader weight_loader(param, loaded_weight, shard_id) break else: # python的for-else语法，到达这里意味着没有执行循环中的 break 语句 # Skip loading extra bias for GPTQ models. if name.endswith(".bias") and name not in params_dict: continue param = params_dict[name] # 根据name找到对应的weight_loader方法 weight_loader = getattr(param, "weight_loader", default_weight_loader) weight_loader(param, loaded_weight)
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模型层权重及其 weight_loader 方法
# param，weight_loader
lm_head.weight, weight_loader <bound method VocabParallelEmbedding.weight_loader of ParallelLMHead()> transformer.h.0.attn.c_attn.weight, weight_loader <bound method QKVParallelLinear.weight_loader of QKVParallelLinear()> transformer.h.0.attn.c_proj.weight, weight_loader <bound method RowParallelLinear.weight_loader of RowParallelLinear()> transformer.h.0.ln_1.weight, weight_loader <function default_weight_loader at 0x7f66201ee0e0> transformer.h.0.ln_2.weight, weight_loader <function default_weight_loader at 0x7f66201ee0e0> transformer.h.0.mlp.c_proj.weight, weight_loader <bound method RowParallelLinear.weight_loader of RowParallelLinear()> transformer.h.0.mlp.gate_up_proj.weight, weight_loader <bound method MergedColumnParallelLinear.weight_loader of MergedColumnParallelLinear()> transformer.ln_f.weight, weight_loader <function default_weight_loader at 0x7f66201ee0e0> transformer.wte.weight, weight_loader <bound method VocabParallelEmbedding.weight_loader of VocabParallelEmbedding()>
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模型的每一层都有自己的分布式加载方法，如 transformer.h.0.attn.c_proj.weight 这个权重使用了 RowParallelLinear.weight_loader 方法。
class RowParallelLinear(torch.nn.Module): def weight_loader(self, param: Parameter, loaded_weight: torch.Tensor): # 获取worker的tp_rank，根据tp_rank计算需要加载的权重范围 tp_rank = get_tensor_model_parallel_rank() input_dim = getattr(param, "input_dim", None) param_data = param.data if input_dim is not None: shard_size = param_data.shape[input_dim] start_idx = tp_rank * shard_size loaded_weight = loaded_weight.narrow(input_dim, start_idx, shard_size) assert param_data.shape == loaded_weight.shape param_data.copy_(loaded_weight)
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模型切分采用了 Megatron-LM 算法，详情可参考论文【文末查看】
四、分布式模型切分算法 Megatron-LM
4.1 分布式节点通信：AllReduce
https://docs.nvidia.com/deeplearning/nccl/user-guide/docs/usage/collectives.html#
1）Reduce：将每个 GPU 的计算结果汇总到某个特定的 GPU 上
2）Broadcast：将某个 GPU 的数据同步到所有 GPU 上
3）AllReduce = Reduce + Broadcast
4.2 Transformer 切分
Transformer 层由一个自注意力模块（self-attention block）后跟一个两层的多层感知机（MLP）实现的。
MLP
如图所示，MLP 由两个部分组成，GeLU 是非线形函数，
即所以不能采用行并行，需要采用列并行。
此时，B 需要采用行并行。如果 B 采用列并行的话，则需要进行一次 all-reduce 同步。
Dropout 是按照一定比例随机丢弃一些参数，因此 Dropout 前必须进行一次 all-reduce 同步。
Self-Attention
multi-head attention 机制中每个 attention 都是独立的 QKV 矩阵，每个 GPU 上计算部分 attention 就行。因此要求 attention head 可以被 tp_size 整除。否则会报错如下（Qwen-14b 设置 tp=3）:
ValueError: Total number of attention heads (40) must be divisible by tensor parallel size (3).
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同样，Dropout 前需要进行一次 all-reduce 操作。
因此，一次 Transformer 推理需要进行 2 次 all-reduce 操作，qwen-14b 中 transformer 有 40 个，一次推理需要执行 81 一个 all-reduce 操作。跨节点部署推理服务时，网络通信将会是比较大的开销。
本文重点分析 vllm 如何实现分布式推理，具体 vllm 的推理过程可参考下方【01 推理过程解析】
参考链接
[01] 推理过程解析
https://zhuanlan.zhihu.com/p/649974825
[02] 【深度学习】【分布式训练】一文捋顺千亿模型训练技术：流水线并行、张量并行和 3D 并行
https://zhuanlan.zhihu.com/p/617087561
[03] Hugging Face 高效训练技术四：多 GPU 分布式训练（DP、PP、TP 、ZeRO）_zero-dp
https://blog.csdn.net/qq_56591814/article/details/134099476
[04] Megatron-LM: Training Multi-Billion Parameter Language Models Using Model Parallelism
https://arxiv.org/pdf/1909.08053
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